During a migration from Google Apps, you’re getting this error:
The user must be signed up for Google Calendar. 
Message[The user must be signed up for Google Calendar.] Location[ - ] Reason[notACalendarUser] Domain[calendar]
The account has a problem with Google authentication.
Please check following issues:
1. User doesn’t exist
2. User is suspended
3. Password has expired
Then restart the migration.
Note: even if you see this error, the platform will still have access to mails and contacts so these items will be migrated successfully.