ajax keep return error instead of sucess

Basically the parameters to be sent must be in JSON format but you are sending the parameter direct value here.

What i think the format should be here is :-

var id = $(this).val();

* * * * * * * * * 

data: "{Id:" + id + "}"

* * * * * * * * * 

Also URL here you sent as 'Servlet', this should be the operation URL not sure what do you mean by 'Servlet' here.

Please change this as mentioned here and check once if it works for you.

Here is a sample ajax call i have pasted below for your help :-
Ex :

$.ajax({
        type: "GET",
        url: "../WebMethods/WebService.asmx/GetData",
        data: "{regId:" + regId + "}",
        dataType: "json",
        async: false,
        success: function (response) {
            return true;
        }
    });

Again you can also use the console window to get the error information like :-

Open the page press F12, developers tool will open
Now click on 'Console' tab and do the operation you want.
Any javascript error can be caught here.