You’re using the wrong OAuth flow. client_credentials creates an App Access Token, meaning it represents the app itself and not a user, and so does not have any user permissions associated with it.
As the developer, you shouldn’t need to be asking users of your service to create their own apps on the TwitchDev console.
You should use your app for the project, and either the Auth Code flow if you want to get tokens that can be refreshed on your server, or the Implicit flow if you want to use the tokens client-side (these can’t be refreshed, so you’d need to send the user through the auth flow each time the token expires). Also, for the endpoint you’re using you’d need the channel:read:redemptions scope, so make sure that’s set on the auth process so that Twitch will explicitly ask the user if they wish to grant that access to your app.