What does "Mailbox has no permission to this action" mean when trying to do ring out via API?

This error occurs when you do not have a phone number ( Direct / Digital line ) assigned to the User / Extension. Free Accounts do not have this capability in Production and they will encounter this error.

See this link on how to set this up: https://devcommunity.ringcentral.com/ringcentraldev/topics/how-to-make-sure-ringout-works