Like a few others that have posted to the forum, I have found I cannot use Meetings APIs with a sandbox account, making it problematic to perform development and, later, to exercise these APIs in order to graduate my app.
In these threads:
Someone from RingCentral has been able to add Meetings to the sandbox user. @Phong Vu if you or someone else could do this for me, I would be eternally grateful. I am a paid user, not just a free developer account.