A Paramount+ subscription with Paramount+ directly isn’t valid for the Apple TV app Channels. It requires a different type of subscription, with different terms and conditions, and different perks and limitations.
To use a Paramount+ subscription from Paramount+ directly, use the separate Paramount+ app. On iPhone, iPad, and Apple TV box, the separate Paramount+ app can be connected to the Apple TV app, for seamless integration in Watch Now, Up Next, and Search (but not the Channel) inside the Apple TV app.
If instead you subscribe to the Apple TV app Channel Paramount+, then Paramount+ offers you an additional complimentary Paramount+ account for use outside the Apple TV app: best of both worlds. To do so, verify your subscription on www.paramountplus.com/apple, and follow the instructions there.