remainder – 403 error PROHIBITED when attempting to insert an item into a SharePoint list

Hi, I'm trying to insert an item from the list into a SharePoint list using the REST API. I receive an error as "{" readyState ": 4," responseText ":" { "error ": { "code ": "- 2130575251, Microsoft.SharePoint.SPException ", "message ": {" lang ": " en-US ", " value ": " The security validation for this page is invalid and may be corrupted. Please use the Back button of your web browser to restart your operation. "}}}", "responseJSON": {"error": {"code": "- 2130575251, Microsoft.SharePoint.SPException", "message": {"lang": " US "," value ":" The security validation for this page is invalid and may be corrupted. Please use the back button of your web browser to retry the operation. "}}}," Status ": 403," statusText ":" PROHIBITED "}"
I used the following code:

function addData () {  

var listUrl = "/ _api / Web / Lists / GetByTitle (& # 39; QuestionsList & # 39;) / Items" ";

var studID = $ ("# stID"). val ();
var optionVal = $ (".rdBtn"). val ();

alert (listUrl);

$ .ajax ({
url: "http: // myserverAndSiteName" + listUrl,

type: "POST",
contentType: "application / json; odata = verbose",
data: JSON.stringify  

({
__metadata:
{
type: "SP.Data.StudentResponseListItem"
}
StudentID: $ ("# stID"). Val (),
Question number: qno,
Question: question,
StudentAnswer: $ (".rdBtn"). Val ()
}),
headers: {
"Accept": "application / json; odata = verbose", // return the data format
"X-RequestDigest": $ ("#__ REQUESTDIGEST"). Val ()
}
success: function (data) {
$ ("# tblEmployees"). empty ();
getItem (listUrl);
alert ("successfully submitted");
}
error: function (error) {
alert (JSON.stringify (error));
}
});
}