sharepoint rest api – how to add value in document library look up column when button is clicked

function UpdateDocsDetails() {
  var def = jQuery.Deferred();
  var itemPayload = "";
  var restSource = _spPageContextInfo.webAbsoluteUrl + "/_api/Web/Lists/getByTitle('DMSDocs')/Items(" + lastItem + ")";
  itemPayload = { "__metadata": { "type": "SP.Data.DMSDocsItem" }, "RequestID": _reqID};
  var dfd = jQuery.Deferred();
  jQuery.ajax(
    {
      url: restSource,
      method: "POST",
      contentType: "application/json;odata=verbose",
      data: JSON.stringify(itemPayload),
      headers:
      {
        "Accept": "application/json;odata=verbose",
        "X-RequestDigest": jQuery('#__REQUESTDIGEST').val(),
        "X-HTTP-Method": "MERGE",
        "If-Match": "*"
      },
      success: function (data) {
        // alert("Success");
        dfd.resolve(data);
      },
      error: function (err) {
        dfd.reject(err);
        // alert("Error:");
      }
    });
  return dfd.promise();
}

this function is need to update the lookup column RequestID

function SaveDeferred(action) {
    var dialog = bootbox.dialog({
      title: 'Saving.......',
      message: '<p>Saving... <i class="fa fa-spin fa-spinner"></i></p>',
      closeButton: false
    });
    var c = CheckForAttachment();
    if (c == true) {
      var p = createItem(action);
      p.done(function (res) {
        if (UpdateFlag == 0) {
          _reqID = res.get_id();
        }
        var d = UpdateDocsDetails();
        d.done(function (_reqID) {
          console.log('Item created successfully!');
          if (UpdateFlag == 0) {
            var _nextNo = GetNextNumber();
            _nextNo.done(function (result___nextNo) {
              console.log(result___nextNo);
              var Series = result___nextNo.d.results(0).Series;
              var StartingValue = result___nextNo.d.results(0).StartingValue;
              var LastUsedValue = parseInt(result___nextNo.d.results(0).LastUsedValue);
              if (LastUsedValue == 0) {
                LastUsedValue = LastUsedValue + 1;
              } else {
                LastUsedValue = LastUsedValue + 1;
              }
              var _update_Title = UpdateTitle(_reqID, Series, LastUsedValue);
              _update_Title.done(function (result__updated) {
                console.log(result__updated);
                var _udp_NoSeries = UpdateNextNumber(Series, LastUsedValue);
                _udp_NoSeries.done(function (result___udp_NoSeries) {
                  dialog.modal('hide');
                  var msg = "Your request Submitted successfully!!";
                  bootbox.alert(msg + " Your request No is " + result__updated, function () {
                    window.location.replace("/sites/" + siteName + "/SitePages/DMSApprovedRequests.aspx");
                  });
                });
                _udp_NoSeries.fail(function (result__udp_NoSeries) {
                  console.log(result__udp_NoSeries);
                });
                console.log("Your request sent successfully!! Your request No is " + result__updated);
              });
              _update_Title.fail(function (result__updated) {
                console.log(result__updated);
              });
            });
            _nextNo.fail(function (result___nextNo) {
              console.log(result___nextNo);
            });
          }
          else {
            dialog.modal('hide');
            bootbox.alert("Your request sent successfully!!", function () {
              window.location.replace("/sites/" + siteName + "/SitePages/DMSApprovedRequests.aspx");
            });
          }
        });
      });
      p.fail(function (ex, error) {
        console.log(ex + " " + error);
      });
    }
    else {
      dialog.modal('hide');
      return false;
    }
}

this function when item saved in SharePoint list but my Page is stuck on saving dialog after UpdateDocs Details() called but not going further to next number update in function