The caller is authenticated but not allowed to do this. When the caller's token is not one this route admits, or does not carry the scope this route requires, the response has no body.
Error codes:
recordAccessDenied — the caller may not read the primary record, or one of the related records, that the request names. field is primaryRecord.id or relatedRecords[n].id.
guestForbidden — the caller holds a guest token that is not a guest-share token.
A guest-share guest may only create an activity whose primary record is the record its share was made for, with no related records.
This endpoint also refuses an activity type that only JobNimbus itself creates. That answer carries no errors array.