Also, what show are you trying to watch?
Apple TV+ (the service) includes access to all Apple-originated materials.
Apple TV (either the app or the black box you connect to your network and TV) acts as a streaming device capable of centrally aggregating content from multiple streaming providers (Apple TV+, Netflix, Hulu, Prime, HBO, etc., etc. etc.) It does not automatically include subscriptions for these other third-party streaming services.
Therefore, while AppleTV may show previews and trailers for shows on other services, you won't be able to view the whole show unless you have a subscription for the respective service.
Identifying the show you're trying to watch should help identify the service provider.