jquery에서 입력 오류 메시지가 표시됨

<form id="signup-form">
  <div class="form-group">
    <label for="name">姓名</label>
    <input type="text" class="form-control" id="name" placeholder="请输入您的姓名">
  </div>
  <div class="form-group">
    <label for="email">电子邮件</label>
    <input type="email" class="form-control" id="email" placeholder="请输入您的电子邮件">
  </div>
  <div class="form-group">
    <label for="password">密码</label>
    <input type="password" class="form-control" id="password" placeholder="请输入密码">
  </div>
  <button type="submit" class="btn btn-primary">提交</button>
</form>

$(document).ready(function(){
  $("#signup-form").validate({
    rules: {
      name: {
        required: true
      },
      email: {
        required: true,
        email: true
      },
      password: {
        required: true,
        minlength: 6
      }
    },
    messages: {
      name: {
        required: "姓名不能为空"
      },
      email: {
        required: "电子邮件不能为空",
        email: "电子邮件格式不正确"
      },
      password: {
        required: "密码不能为空",
        minlength: "密码至少需要6个字符"
      }
    },
    errorElement: "div",
    errorPlacement: function(error, element) {
      error.addClass("invalid-feedback");
      element.closest(".form-group").append(error);
    },
    highlight: function(element, errorClass, validClass) {
      $(element).addClass("is-invalid").removeClass("is-valid");
    },
    unhighlight: function(element, errorClass, validClass) {
      $(element).removeClass("is-invalid").addClass("is-valid");
    }
  });
});