Did you correctly follow the documentation:
To refresh, use this request:
curl -X POST https://api.twitch.tv/kraken/oauth2/token
--data-urlencode
?grant_type=refresh_token
&refresh_token=<your refresh token>
&client_id=<your client ID>
&client_secret=<your client secret>
Sounds like you have performed a GET request instead of a POST request