Unable to disable and enable button based on condition

Question

[HttpGet]
public ActionResult ButtonValidation(int? id)
{
var result = false;
var items = db.VW_PLAN_TY_STG.Where(x => x.JOB_BTCH_ID == id ).FirstOrDefault();
if (items.STAT_DESC == "Error")
{
result = true;
}
return Json(new { emailSent = result });
}

this.btnvalidate = function(jobBatchId) {
var url = '@Url.Action("ButtonValidation")/' + jobBatchId;
var promise = $.ajax({
url: url,
type: 'GET',
//data: jobBatchId,
dataType: 'JSON',
processData: true,
contentType: "application/json; charset=UTF-8",
cache: false,
success: function (data) {
if (data.emailSent == 'true') {
$('#btnSubmit').prop('disabled', true);
} else {
$('#btnSubmit').prop('disabled', false);
}
}
});