Google Calendar sample gives 503 error (Service Unavailable) when deployed

I am trying to play with the example at Building conferencing add-ons and Building conferencing add-ons sample.

I deployed my example conferencing add-on to my google account and tried to test. But It was failed to create conference and give me 503 error (Service Unavailable).

Actually I am beginner in Google Conference Add-Ons. It says 'The service is currently unavailable'. Is that right? Or Do I have to do something more to make it available?

Thank you.