以下代码中没有出错

Question

这是我的前端页面，在这里我想显示错误。

<div class="error" style="color:red; text-align:center;"></div>

这是我的后端php验证码：

<?php
$error = array();

$user_id=$_SESSION['email'];
$apply_id=$_SESSION['id'];      

    $purpose_amount=$_POST['purpose_amount'];
    $amount=$_POST['amount'];
    $cheque_name=$_POST['cheque_name'];

    if(empty($amount)){
        $error[] =  "Please Enter Expected Amount";
    }
    elseif(empty($cheque_name)){
        $error[] =  "Please Enter Issue Cheque in Favour of";
    }
    if (count($error)) {
        echo "<i class='fa fa-exclamation'> ";
        echo implode('<br>', $error);
        echo "</i>";
    }
else{
    $result=mysqli_query($conn, "UPDATE education SET amount='$amount', cheque_name='$cheque_name', purpose_amount='$purpose_amount', date='$date' WHERE id='$apply_id'")or die("Could not insert data: " .mysqli_error($conn));
        echo $user_id;          
}
?>

这是Ajax和Jquery代码：

$('.btn_education_step5').click(function(){            
    var formdata=new FormData($('#education_step5')[0]);         
    $.ajax({
            url:'includes/backend_education_amount.php',
            method: "POST",
            async: false,
            cache: false,
            contentType: false,
            processData: false,
            data : formdata,
        success:function(answer_from_actionpage){
            window.location.href = 'education_detail.php?uid='+answer_from_actionpage;           
        }
        error:function(answer_from_actionpage){
            $('.error').html(answer_from_actionpage);        
        }
    })
}); 

当字段为空时，我尝试显示错误。但没有得到回应会发生什么错误。