You’re requesting an App Access token, you need to use a User Access Token.
App access tokens are used for requests that aren’t on behalf of a specific user, in your case you’re trying to access the bits leaderboard for ‘thebubbaarmy’ so you need to request a User access token and have that user go through your authentication process, which will give you a token valid to do the request for the bits leaderboard on their channel.
As a side note, you’re also performing this request incorrectly as it needs the users ID, not username.