Problem Statement:
Gmail and GDrive is not backing up with error message ‘EUSERNOTFOUND’
Cause:
The ‘Cloud App Settings’ is set to use UPN.
Google Workspace does not support UPN and works on email id.
Resolution:
Go to the Google Workspace Overview Page
Click on the 3 dots on the right corner>Settings
Change the 'Cloud App Settings' to 'inSync Email ID'.