SpreadsheetApp.openById gives unauthorised error

I am trying to trigger a function in Google Apps Scripts, and am using openById("Sheet_Id);But on using it, I am getting this error:

Exception: You do not have permission to call SpreadsheetApp.openById. Required permissions: https://www.googleapis.com/auth/spreadsheetsat saveActiveSheet(Code:2:27)at onSelectionChange(Code:27:9)