Your problem is likely the scope that is granted to the OAuth token you are using. The one obtained from twitchapps.com/tmi will not work as the only provides the “chat_login” scope.
If you are requesting GET /users/:user/subscriptions/:channel when you are required to provide a token with the “user_subscriptions” scope on behalf of :user.
You can read more about the authentication process here:
https://github.com/justintv/Twitch-API/blob/master/authentication.md