It seems the service in sandbox environment is down. I made a test call and got the same error. I just report the incident to the team. Hope it will get fixed soon.

@Phong Vu the error "{ "errorCode": "MET-106", "message": "Cannot create meeting. Reason : [Remote service unavailable]" }" is not yet fixed. can you fix this issue as soon as possible.

I have a ticket for the issue but it has not been picked up yet. All I can help you with for now is to manually graduate your app to the production so you can test it on the your production account. If you want to do so, provide me with the app client id.

Remember that even if the issue is fixed, there is no actual meeting supported in sandbox. This means that you can call the API to create/schedule a meeting, but there is no actual server to host the meeting for sandbox. So the API is just for satisfying the API permission for graduation.