Welcome to the Auth0 Community!
I understand that you have encountered the 401 Unauthorized error when trying to get a new access token using a refresh token.
After testing this myself, I could successfully get a new access token using a refresh token. The only way I could reproduce the 401 Unauthorized error is when I used an incorrect client_id and/or client_secret in my request.
Given that, could you please double-check that your client_id and client_secret values are correct?
Please let me know your findings.
Thank you.