Hi Juanito956,
Depending on the exact prompt you're seeing, the steps below could help you to Set up the Fitness app on Apple TV with your Apple Watch or iPhone:
"Choose who's working out
If you use Family Sharing, you can easily switch between family members in the Fitness app on Apple TV. Friends or family outside of your Family Sharing circle who are Apple Fitness+ subscribers can also work out using your Apple TV.
- Open Fitness
on Apple TV. - Choose your name, or choose Other if you don’t see your name.
- If no one is signed in on your Apple TV, you may need to open the Workout app on Apple Watch or open the Fitness app on iPhone first (requires watchOS 7.2 or iOS 16.1 or later).
- To switch to another family member or guest, for example, when you finish working out, select the account icon in the top left corner of the Fitness app, choose Sign Out, then choose another user."
We hope that helps. Take care.