This error occurs when calendars for mailboxes that are being migrated have been turned off in the Google Admin Console, or the OAuth2 API Scopes have not been entered by the GSuite Admin.
It is best to contact your customer to determine if the calendar services were turned off, the API Scopes entered as needed, and if the calendars for the affected mailboxes need to be migrated. If the calendars need to be migrated, then the service and API Scopes must be active.
Read the following articles on activating the calendar service and entering the API Scopes:
You can retry the migration once the above is confirmed.