question

Making this conversation private since you've shared your API keys and user credentials via the graphic (and we wouldn't want someone to hack your account with this information).

I see you're trying to hit the Production API Endpoint URI: https://platform.ringcentral.com/restapi/oauth/token are your keys being used for that environment or are they for the Sandbox API Endpoint URI: https://platform.devtest.ringcentral.com/restapi/oauth/token ?

Are the user credentials being used associated with the appropriate environment?

Before testing the service, following things to be checked:

correct client I'd and secret, username should be phone number and extention number should be correct and allowed permission and correct url based on environment like sandbox/ production