My CSS stylesheet no longer works/links to my HTML file - what could be the reason?-PHP Chinese Network Q&A

I've searched many forums for answers and looked at about 50 google links but haven't found anything that works yet...

My CSS is not showing up on my HTML.

My code was working fine until I added a Flexbox container and 2 Flexboxes in it.

I always test my code so I know Flexbox will work outside of this specific HTML file.

Also, I noticed that my CSS was not updating on "Inspect Element" on localhost/#my_file#. This has happened in the past and I solved it by changing the file location and changing it again to the old location. (I don't know how rhat works).

Anyway, if I copy and paste the CSS update table onto the "Inspect Element" page, I can view my Flexbox just fine.

So I suspected it was because of a bug in my CSS, but I wasn't quite sure, so I even checked it: https://jigsaw.w3.org/css-validator/#validate_by_input

There is only 1 error, but I checked it in the entire CSS file and 1) I couldn't find it 2) I removed everything that looked most like the error and it still doesn't work.

Obviously, I don't think this has anything to do with the error, since it works just fine with "inspecting the element".

I also know that I make careless mistakes, stupid mistakes, so...you know.

I'm currently studying, if you find any bugs in my code, please let me know!

HTML：

<!DOCTYPE html>
<html lang="fr">
<head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Darki0ni-s-website</title>
    <link href="https://fonts.googleapis.com/css?family=Josefin+Slab" rel="stylesheet" type="text/css">
    <link href="https://fonts.googleapis.com/css?family=Josefin+Sans" rel="stylesheet" type="text/css">
    <link href="styles/portfolio.css" rel="stylesheet" type="text/css">
    <script src=
    "https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js">
        function validateForm()                                    
{ 
    var name = document.forms["myForm"]["name"];               
    var email = document.forms["myForm"]["email"];    
    var message = document.forms["myForm"]["message"];   
   
    if (name.value == "")                                  
    { 
        document.getElementById('errorname').innerHTML="Please enter a valid name";  
        name.focus(); 
        return false; 
    }else{
        document.getElementById('errorname').innerHTML="";  
    }
       
    if (email.value == "")                                   
    { 
        document.getElementById('erroremail').innerHTML="Please enter a valid email address"; 
        email.focus(); 
        return false; 
    }else{
        document.getElementById('erroremail').innerHTML="";  
    }
   
    if (email.value.indexOf("@", 0) < 0)                 
    { 
        document.getElementById('erroremail').innerHTML="Please enter a valid email address"; 
        email.focus(); 
        return false; 
    } 
   
    if (email.value.indexOf(".", 0) < 0)                 
    { 
        document.getElementById('erroremail').innerHTML="Please enter a valid email address"; 
        email.focus(); 
        return false; 
    } 
   
    if (message.value == "")                           
    {
        document.getElementById('errormsg').innerHTML="Please enter a valid message"; 
        message.focus(); 
        return false; 
    }else{
        document.getElementById('errormsg').innerHTML="";  
    }
   
    return true; 
}
        </script>
</head>
<body>
    <script>
        $(document).ready(function () {
  
            $("a").on('click', function (event) {
  

                if (this.hash !== "") {
  
                    event.preventDefault();
  
                    var hash = this.hash;
  
                    $('html, body').animate({
                        scrollTop: $(hash).offset().top
                    }, 500, function () {

                        window.location.hash = hash;
                    });
                }
            });
        });
    </script>
        <header class="header">
            <article id="accueil"></article>
            <div id="header">
                <div id="background">
                    <article style="margin-bottom: 175px;" ><br></article>
                    <h1>MY NAME</h1>
                    <h4>___________________</h4>
                    <h2>Développeuse web débutante</h2>
                </div>
            </div>
        </header>
        <ul class="menu">
            <li>
              <a href="#a-propos">À Propos</a>
            </li>
            <li>
              <a href="#competences">Compétences</a>
            </li>
            <li>
              <a href="#contact">Contact</a>
            </li>
            <li>
              <a href="#accueil" class="actif">&#69716;</a>
            </li>
        </ul>
          <p id="a-propos"></p>
    <h3>À Propos de moi</h3>
    <div class="container">
        <div class="flexbox-left">
            <p>Qui suis-je ?</p>
            <p>blablabla </p>
        </div>
        <div class="flexbox-right">
            <p>blablabla <br>
                blabla <br> 
            </p>    
        </div>
    </div>
         <p id="competences"></p>
        <h3>Compétences</h3>
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Sagittis nisl rhoncus mattis rhoncus urna neque viverra. Lectus magna fringilla urna.</p>
        <p id="contact"></p>
        <h3>Contact</h3>
        <div class="container">
        <form name="myForm" action="mail.php" onsubmit="return validateForm()" method="post">
            <table class="form-style">
               <tr>
                  <td>
                     <label for ="name">
                        Your name <span class="required">*</span>
                     </label>
                  </td>
                  <td>
                     <input type="text" name="name" class="long" id="name"/>
                     <span class="error" id="errorname"></span>
                  </td>
               </tr>
               <tr>
                  <td>
                     <label for ="email">
                       Your email address <span class="required">*</span>
                     </label>
                  </td>
                  <td>
                     <input type="email" name="email" class="long" id="email"/>
                     <span class="error" id="erroremail"></span>
                  </td>
               </tr>
               <tr>
                  <td>
                     <label for ="message">
                        Message <span class="required">*</span>
                     </label>
                  </td>
                  <td>
                     <textarea name="message" cols="50" rows="10" id="message"></textarea>
                     <span class="error" id="errormsg"></span>
                  </td>
               </tr>
               <tr>
                  <td></td>
                  <td>
                     <input type="submit" value="Send">      
                     <input type="reset" value="Reset"> 
                  </td>
               </tr>
            </table>
         </form>
      </div>
         <p>Bonjour et bienvenue sur mon site web dévellopé from scratch, il est encore simple mais il représente mon avancée en terme de dévellopement Front End. </p>
         <footer>
    </footer>
</body>
</html>

CSS：

html {
  margin: 0px 0px;
  padding: 0px 0px;
  background-color: #cab4cb;
}

body {
  font-family:'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  width:auto;
  height: auto;
  margin: 0px 0px;
  background-color: #f15015;
  padding: none;
  border: none;
}

header {
  overflow: hidden;
  padding: 0px 0px;
  height:auto;
  background-image:url(../images/6.jpg);
  background-position-y: top;
  background-position:center;
  background-size: cover;
  border:none;
  width:auto;
  margin:0px 0px;
  position:static;
}

.background {
  background: linear-gradient(rgba(0, 0, 0, 0.7), rgba(0, 0, 0, 0.7));
  background-blend-mode: darken;
  height: 560px;
  width: auto;
  border: none;
  padding: 25px 0px;
  margin: 0px 0px;
  background-repeat: repeat;
  background-size: cover;
  }

p {
  margin-left: 15%;
  margin-right: 15%;
  padding-top: 2%;
  text-align: center;
  font-family:'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  font-size: 16px;
  line-height: 2;
  letter-spacing: 1px;
}

h1 {
  text-align: center;
  font-family: 'Josefin Sans', sans-serif;
  font-size: 45px;
  font-weight: lighter;
  margin: 0px 0px;
  padding: 0px 0px;
  color: #dadada9a;
  text-shadow: 2px 2px 1px #0f0f0f95;
  position:relative;
}

h2 {
  text-align: center;
  font-family: 'Josefin Slab', sans-serif;
  font-size: 27px;
  font-weight: lighter;
  margin: 0px 0px;
  padding: 30px 0px;
  color: #cdcdcd9a;
  text-shadow: 2px 2px 1px #0f0f0f95;
}

h3 {
  text-align: center;
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  font-size: 25px;
  font-weight:normal;
}

h4 {
  text-align: center;
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  font-size: 25px;
  font-weight:normal;
  color: #cdcdcd9a;
}

img {
  height: auto;
  object-fit: cover;
  display: block;
  margin: 0 auto;
}

footer{
    padding:25px 0px;
    background-color:#cdcdcd9a;
    text-align:center;
}

.menu {
  display:flex;
  background-color: #dddd;
  position: -webkit-sticky; /* Safari */
  position: sticky;
  top: 0;
  width: 100%;
  margin: 0;
  padding: 0;
  overflow: hidden;
  justify-content: space-around;
}

.menu li {
  list-style-type: none;
  float: left;
  display: inline;
  font-size: 16px;
  line-height: 2;
  letter-spacing: 1px;
  float: right;
}

.menu a {
  display: block;
  padding: 8px;
  display: block;
  color: rgb(120, 120, 120);
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
  transition: all 1s ;
}

.menu a.actif {
  background-color: rgba(120, 120, 120, 0.428);
  color: #dddd;
  font-size: 16px;
}

.menu a:hover,
.menu a:hover.actif{
  background-color: rgb(31, 31, 31);
}

.form-style {
  margin:10px auto;
  width: 400px;
  padding: 20px 12px 10px 20px;
  font: 14px "Lucida Sans Unicode", "Lucida Grande", sans-serif;
}
.form-style td {
  padding: 0;
  display: block;
  list-style: none;
  margin: 10px 0 0 0;
}
.form-style label{
  margin:0 0 3px 0;
  padding:0px;
  display:block;
  font-weight: bold;
}
.form-style .required{
  color:rgb(255, 0, 128);
}
.form-style input[type=submit], .form-style input[type=reset]{
  background: #cdcdcd9a;
  padding: 8px 15px 8px 15px;
  border: none;
  color: #fff;
}
.form-style input[type=submit]:hover, .form-style input[type=reset]:hover{
  background: #cdcdcd9a;
  -moz-box-shadow:none;
  -webkit-box-shadow:none;
  box-shadow:none;
}
.form-style .field-textarea{
  height: 100px;
}
.form-style input[type=text], 
.form-style input[type=email],
textarea{
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  border:1px solid #BEBEBE;
  padding: 7px;
  margin:0px;
  -webkit-transition: all 0.30s ease-in-out;
  -moz-transition: all 0.30s ease-in-out;
  -ms-transition: all 0.30s ease-in-out;
  -o-transition: all 0.30s ease-in-out;
  outline: none;  
}
.form-style .long{
  width: 100%;
}
.form-style input[type=text]:focus, 
.form-style input[type=email]:focus,
.form-style textarea:focus{
  -moz-box-shadow: 0 0 8px #cdcdcd9a;
  -webkit-box-shadow: 0 0 8px #cdcdcd9a;
  box-shadow: 0 0 8px #cdcdcd9a;
  border: 1px solid #cdcdcd9a;
}
.error{
  color: #D8000C;
  background-color: #FFBABA;
}

.container {
  display: flex;
  flex-direction: row;
  margin-left:200px;
  margin-right:200px;
  margin-top:100px;
  margin-bottom :100px;
  background-color: rgb(110, 76, 110); 
  justify-content:center;
}

.flexbox-left {
  flex: 100%;
  padding: 25px 0;
  width:auto;
  height:auto;
  text-align: center;
  border:2px solid rgb(85, 21, 73);
  background-color: rgb(254, 199, 255);
}

.flexbox-right {
  flex: 100%;
  padding: 25px 0;
  width:auto;
  height:auto;
  text-align: center;
  border:2px solid rgb(85, 21, 73);
  background-color: rgb(254, 199, 255);
}

.a-propos {
  padding: 30px;
  margin: 0px;
}

.competences {
  padding: 30px;
  margin: 0px;
}

.contact {
  padding: 30px;
  margin: 0px;
}