JQuery AJAX说加载函数不是函数

When I press the submit button for my form, it calls JQuery to stop the page from refreshing, but it says that the function is not found (load).

I get multiple errors in the console, here is a screenshot: https://gyazo.com/ed25b9f8c1c8b0a6ecafedbad4d4e9ef

    <!doctype html>
<html lang="en">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" integrity="sha384-9gVQ4dYFwwWSjIDZnLEWnxCjeSWFphJiwGPXr1jddIhOegiu1FwO5qRGvFXOdJZ4" crossorigin="anonymous">

    <title>TheTawk | Sign up</title>

    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.0/jquery.min.js"></script>

    <script type="text/javascript">
      $(document).ready(function(){
        $('form').submit(function (event){
          event.preventDefault();
          var firstName = $("#firstName").val();
          var lastName = $("#lastName").val();
          var email = $("#email").val();
          var username = $("#username").val();
          var password = $("#password").val();
          var signUp = $("#signUp").val();
          $(".form-message").load("classes/register",{
            firstName: firstName,
            lastName: lastName,
            email: email,
            username: username,
            password: password,
            signUp: signUp
          });
        });
      });
    </script>
  </head>
  <body>
    <?php include 'includes/header.php'; ?> <br><br>

    <div class="container">

      <div class='row justify-content-md-center'>
        <div class='col-md-auto'>
          <form autocomplete="off" action="" ="classes/register" method="post">
            <input type="text" class="form-control" name="firstName" id="firstName" autofocus="on" placeholder="First name"><br>
            <input type="text" class="form-control" name="lastName" id="lastName" placeholder="Last name"><br>
            <input type="text" class="form-control" name="email" id="email" placeholder="Email"><br>
            <input type="text" class="form-control" name="username" id="username" placeholder="Username"><br>
            <input type="password" class="form-control" name="password" id="password" placeholder="Password"><br>
            <button type="submit" class="btn btn-primary" name="signUp" name="signUp" style="width: 100%;">Sign up</button>
            <p class="form-message"></p>
          </form>
        </div>
      </div>

    </div> 

    <?php include 'includes/footer.php'; ?>

    <!-- Optional JavaScript -->
    <!-- jQuery first, then Popper.js, then Bootstrap JS -->
    <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js" integrity="sha384-cs/chFZiN24E4KMATLdqdvsezGxaGsi4hLGOzlXwp5UZB1LY//20VyM2taTB4QvJ" crossorigin="anonymous"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js" integrity="sha384-uefMccjFJAIv6A+rW+L4AHf99KvxDjWSu1z9VI8SKNVmz4sk7buKt/6v9KI65qnm" crossorigin="anonymous"></script>
  </body>
</html>

Thanks, Ethan!