ajax错误500（内部服务器错误）

Question

我使用ajax将数据表单传递给laravel控制器 但是当我运行我的代码时，它总是返回错误（内部服务器错误）

    $(document).ready(function() {
    $("#btnSave").click(function(e){
        var _token = $("input[name='_token']").val();
        // var selPartners = $("input[name='selPartners'] option:selected").val();
        var selPartners= $('#selPartners :selected').text();
        var txtDescription = $("input[name='txtDescription'] ").val();
        var txtPrice = $("input[name='txtPrice']").val();
        var txtWarrantyDate = $("input[name='txtWarrantyDate']").val();
        var itemImage = $('#itemImage').prop('files')[0];
        // var selItems =$("input[name='selItems']").val();
        var selItems= $('#selItems :selected').text();
        var x ={_token:_token, selPartners:selPartners, selItems:selItems, txtDescription:txtDescription, txtPrice:txtPrice, txtWarrantyDate:txtWarrantyDate, itemImage:itemImage}
        // console.log(x);

        $.ajax({
            url: "/additem",
            type:'POST',
            dataType: 'json',
             cache: false,
            contentType: false,
            processData: false,
            data: {_token:_token, selPartners:selPartners, selItems:selItems, txtDescription:txtDescription, txtPrice:txtPrice, txtWarrantyDate:txtWarrantyDate, itemImage:itemImage},
            success: function(data) {

                if($.isEmptyObject(data.error)){
                    console.log('ok');

                }else{
                    console.log('failed');
                }
            }
        });
    });
});

在控制器上只有：

return response()->json(['error'=>'failed']);