我的Ajax表单提交不起作用

我的表单无法工作。即使查询没有像我希望用户名是唯一的那样执行，它也会成功，所以我想在用户名不唯一时触发一个错误，如果成功，我想重定向到一个页面，即"clients.php"，所以这是我的表单代码，请为：

<script src="js/jquery-1.11.0.js"></script>
<script>
$(document).ready(function(){
    $('#add_client').on('submit',function(e) {
        $.ajax({
          url:'add_client_query.php',
          data:$(this).serialize(),
          type:'POST',
          success:function(data, textStatus) {
            console.log(data);     
            window.location.href = "clients.php";    // Show Success Message
          },
          error:function(data) {
            $("#error").show().fadeOut(5000);    // Show Error Message
          }
        });
        e.preventDefault();    // To Avoid Page Refresh and Fire the Event "Click"
    });
});
</script>

这是HTML表单代码：

<div class="alert alert-danger" id="error" style="display:none;">One Account Is Already Exist There With The Username.Please Try With Unqiue Username.</div>
<form role="form" action="add_client_query.php" method="post" id="add_client">
  <div class="form-group">
    <label>Username :</label>
    <input name="username" class="form-control">
  </div>
  <div class="form-group">
    <label>Password :</label>
    <input name="password" class="form-control">
  </div>
  <div class="form-group">
    <label>Assign Form</label>
    <select class="form-control" name="formlink">
      <?php    // $resultset now holds all rows from the first query.
      foreach ($resultset as $result) {  ?>
      <option value="<?php echo $result['form_name']; ?>"><?php echo ucwords($result['form_name']); ?></option>
      <?php } ?>
    </select>
  </div>
  <div class="form-group">
    <label>Do You Want To Assign Invoice To The Client?</label></br>
    <label><input type="radio" id="yes" name="invoice_choice[]" id="optionsRadios1" value="Yes"> Yes</label>
    <label><input type="radio" id="no" name="invoice_choice[]" id="optionsRadios1" value="No"> No</label>
  </div>
  <div class="invoice_price" style="display:none;">
    <div class="form-group" >
      <label>Invoice Price :</label>
      <input name="invoice_price"  class="form-control">
    </div>
  </div><!-- end div invoice_price -->
  <button type="submit" class="btn btn-default">Submit Button</button>
</form>

下面是用于查询的PHP代码：

<?php
include('../config/config.php');
session_start();
if ($_SESSION['admin']) { 
    if (isset($_POST['username'])) {
        $username = $_POST['username'];
        $password = $_POST['password'];
        $formlink = $_POST['formlink'];
        $invoice_choice = $_POST['invoice_choice'];
        foreach ($invoice_choice as $invoice_choice) 
        { }
        if ($invoice_choice=="Yes") {
            $invoice_price = $_POST['invoice_price'];
            $invoice_status = "Pending";
        } elseif ($invoice_choice=="No") {
            $invoice_price = "0";
            $invoice_status = "Not Assigned";
        }
        $query = mysqli_query($mysqli,"INSERT INTO clientlog  (Username,Password,formlink,invoice_price,invoice_status)
        VALUES ('$username','$password','$formlink','$invoice_price','$invoice_status');") or die(mysqli_connect_error());
    }
} else {
    header('Location:index.php');
} 
?>

所以，如果你们能告诉我哪里做错了什么，那将是一件非常好的事情。