问浏览器控制台中的ajax错误-不是一个函数

EN

<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.3.1.js"></script>
<input type="button" id="id_name_task" value="Click" />


<script type="text/javascript">
  $(document).ready(function() {
    $("#id_name_task").click(function() {
      var name_task = $(this).val();
      console.log(name_task);
      $.ajax({
        type: 'GET',
        async: true,
        dataType: 'json',
        url: '/validate_data/',
        data: {
          'name_task': name_task
        },
        success: function(data) {
          if (data.is_taken) {
            alert("A task with this name already exists.");
          }

        },
      });
    });
  });
</script>