This error occurs when calendars for mailboxes that are being migrated either are not accessible or have been turned off in the Google Admin Console. Contact your customer to verify that API access is enabled for the Google Admin account as described in the article "Cannot connect to Admin API - Users: Google.Apis.Requests.RequestError Domain cannot use APIs". If API access is already enabled, determine if the calendars were turned off and if the calendars for the affected mailboxes need to be migrated. If the calendars need to be migrated, then they must be turned on. Read the Turn Calendar on or off for users article from Google for more information. Retry the migration for the affected mailboxes once the calendars have been turned on.