You can’t use http://localhost as a callback, you need to use an address that is internet accessible and has a web server running that will listen for the GET and POST messages sent by Twitch.
Once you send the subscription request to Twitch, they will then send a GET request to your callback (and since you’re using localhost, you’ll never receive it as that’s not reachable by Twitch) at which point you need a web server running to listen for that request and respond with the hub.challenge contained in the request.
After you respond to that challenge the webhook subscription should be created, and you will be sent POST requests to your callback which contain the notifications about the topic you requested.