The error seems to reflect that your credentials are wrong. What is the URL you are hitting? Is that dev or prod?
Can you please check with developer support and submit a help ticket with your application client id so that they can check the issue?
Is this a free tier app?
The last thing now I can guess is being a free tier app.
As per my conversation with the backend dev support team was when I was facing similar kind of issues not able to test the app in production.
What they said was free tier app will not be able to test it using our account and any other RC user (paid user) can test the integration app in production.
Now I hope this is not the case with you.
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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