I was able to reproduce your error by NOT passing grant_type=refresh_token in the body of the request.
I get back the accessToken if I include grant_type=refresh_token.
Just to be clear, you are able to get the Authorization Code and exchange it for access and refresh tokens right?
For the first /token request, you pass grant_type=authorization_code and you will get back access/id and refresh tokens.
Now for the second /token request, you pass the request token with grant_type=refresh_token and get back a new access token.