Can’t login to app on Apple TV

Devices you can use with Apple Fitness+ - Apple Support

You will need an Apple Watch series 3 or later with with watchOS 7.2 or later, as well as an Apple TV set-top box (4th generation or later) with tvOS 14.3 or later. Obviously in addition to the Apple Fitness+ subscription itself.

Do you have that? Then you would find the Fitness app on the home screen.

Work out on Apple TV - Work out with Apple Fitness+ - Apple Support

Select yourself in the Fitness app on Apple TV. Select Other if you're not signed in as the primary account and don't have your Apple Watch paired with the Apple TV. To pair your Apple Watch, open the Workout app on your watch and enter the code displayed on the Apple TV.

If you see "Pairing Canceled" while starting a workout in Apple Fitness+ - Apple Support

If you open Apple Fitness+ on your Apple TV and see "Pairing Canceled" when you try to connect your Apple Watch, learn what to do.