I am sure that the problem is about authentication. I am not familiar with MS power automate so I cannot help you in that part. But if you can share the type of your RC SMS application auth flow (code flow or JWT flow) and how do you authenticate a user to get the access token for calling the /sms API then I maybe able to say what could be wrong.
I downloaded the Postman collection from Github, which is from the instructions in the RC developer/API guide.
I used, POST Get access token- by personal JWT, under Authorization Flows dropdown.
This is a JWT flow. In Postman, I use:
Post: https://platform.devtest.ringcentral.com/restapi/oauth/token
Authorization type is Basic Auth
Username: My client_ID
Password: My client_secret
Hope the image help. Thanks.