Refresh Token

I'm trying to work out how to get a refresh token from the API.
I have no problems getting an access token with - Authentication (OAuth 2.0) - Client Credentials.
The following method appears to be the only way to get a refresh token, is that correct?
However when I try to use - Authentication (OAuth 2.0) - Resource Owner Credentials
I get "error": "unsupported_grant_type" The grant type was "password" as described in the documentation.
I get this error in both Postman and Python.
Any assistance would be much appreciated