It sounds as if you've activated a Sky Discovery+ account linked to your Sky sub, (may be a linked free account). Therefore TNT Sport will not be available. You can check by opening the Discovery+ app, go to settings, and click on subscription, that will confirm who your sub is linked to. If it is Sky then you will need to sign out and enter your BT details instead. Mind there could be a problem if you're using the same email for both subs.
