Fitness not working with Airplay 2

AirPlay is not supported.

Fitness+ List of supported devices => Devices you can use with Apple Fitness+ - Apple Support

FYI: Anything not in the list is not supported.

What Apple Watch do you have?

Yes, you need one. Apple Watch personalizes your Apple Fitness+ experience by integrating your personal metrics, like your heart rate and the calories you burn, into every workout. It brings them to life on the screen to keep you informed and motivated. To use Apple Fitness+ on your iPhone, iPad, or Apple TV, an Apple Watch Series 3 or later is required.

Apple Fitness+ is available in

1. The United States
2. Canada
3. Australia
4. New Zealand
5. The United Kingdom
6. Ireland

You don't, if you do not have an TV HD/4K running tvOS 14.3 as stated in the support document.

It will also not run on TV 1st, 2nd and 3rd generation (cannot update to the required tvOS 14.3)

There's nothing to fix.

Apple provided the list of required hardware (Watch, iPhone, iPad and TV) as well as the required iOS/iPadOS/tvOS 14.3 to use Fitness+.

AirPlay 2 is not supported.

Feedback to Apple here => https://www.apple.com/feedback

As has been stated several times on here Airplay 2 is not supported. No one here works for Apple and we are just users like yourself. The only thing you can do is send Feedback in the provided link above and maybe Apple will add it in the future.

You do not have to buy an TV, if you have an iPhone or an iPad running the require OS version.

If not you missed out.