This only works with the providers that support Free/Busy, such as Google and Microsoft EWS. Graph and EAS are not currently supported.
The response should an unsupported provider be detected:
Nylas: Exception while create_calendar_event, , exception: 400 Bad Request. Reason: Enhanced events are not supported for this account type.. Nylas Error Type: invalid_request_error
check_slot_status: 400 Bad Request. Reason: Free-busy for `emails` is not supported by the EAS provider.. Nylas Error Type: invalid_request_error