PHP에서 '치명적인 오류: 'PHPMailer' 클래스를 찾을 수 없음' 오류가 발생하는 이유는 무엇입니까?

include_once('C:\Inetpub\wwwroot\php\PHPMailer\PHPMailerAutoload.php');

require("/home/site/libs/PHPMailer-master/src/PHPMailer.php");
require("/home/site/libs/PHPMailer-master/src/SMTP.php");

$mail = new PHPMailer\PHPMailer\PHPMailer();

    $mail->IsSMTP(); // enable SMTP
    $mail->SMTPDebug = 1; // debugging: 1 = errors and messages, 2 = messages only
    $mail->SMTPAuth = true; // authentication enabled
    $mail->SMTPSecure = 'ssl'; // secure transfer enabled REQUIRED for Gmail
    $mail->Host = "smtp.gmail.com";
    $mail->Port = 465; // or 587
    $mail->IsHTML(true);
    $mail->Username = "xxxxxx";
    $mail->Password = "xxxx";
    $mail->SetFrom("[email protected]");
    $mail->Subject = "Test";
    $mail->Body = "hello";
    $mail->AddAddress("[email protected]");

     if(!$mail->Send()) {
        echo "Mailer Error: " . $mail->ErrorInfo;
     } else {
        echo "Message has been sent";
     }