Sheets function working in standalone Google Script but NOT when script is bound to a Google Sheet

I have this piece of code that is working perfectly when I use it in a standalone Google Script running as a web-app :

function getSubjectsRollNosFromGradeSheet() {
  var spreadsheetId = '171hWWH8bV6kM1JJ2dvL80q3BoWC6eEc0U7l7wCTaXHE';
  var rangeSubject = 'GradesT1!G1:AA1';

  var values = Sheets.Spreadsheets.Values.get(spreadsheetId, rangeSubject).values;

However when I write this same code in a Script that is embedded inside another Google sheet (not the one being accessed for data) attached to a Google Form I get an error :

function updateForm(){

  // call your form and connect to the drop-down item
  var form = FormApp.openById("1VV_LeFJODZcIzfcYOh8vHEkhl033Lx4ZsXaY4Pm1ZUE");
  var subList1 = form.getItemById("497738674").asListItem();

  //pull data from another sheet
  var spreadSheetID = "1EJSx62rZuLSKIgel7LH8hE7-68lupz4buutmVQBqi2I";
  var rangeSubs = 'Sheet1!A1:A';

  var subValues = Sheets.Spreadsheets.Values.get(spreadSheetID, rangeSubs).values;

I get this error

ReferenceError: "Sheets" is not defined. (line 16, file "Code")

Why is the Sheets object available in the first case but not in the second case and how should I change my code to overcome this error.