Same problem here:
Sending SMS using API gives error code MSG-242 "The requested feature is not available"
I just setup a sandbox and trying to send SMS using the API
Using the RingCentral Phone for Desktop, you can dial or receive test calls, send and receive test SMS or Fax messages in your sandbox environment.
Download RingCentral Phone for Desktop:
Tip: switch to the "sandbox mode" before logging in the app:
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