For channel subscriptions on pubsub, you will need a User Access token with the channel_subscriptions scope. App access tokens aren’t for accessing anything that requires user authorization.
To get a User Access Token you can follow the documentation https://dev.twitch.tv/docs/authentication/
You need the owner of the channel you want to get events for to go through the authentication flow for your app and that will provide you with a User Token that you can use to get subscriber events for their channel. For bits any scope will work, you don’t need a specific one, but it does have to be from the channel owner of the channel you want events for.
Alternatively you can use an IRC or Websocket connection to Twitch Chat and listen to events through that https://dev.twitch.tv/docs/irc/ but again you will need permission from the channel owner to do so or you could be breaking the Developer Agreement.