Apple Watch is Unavailable on Apple Fitness +

You didn't say what series watch you have. You must have an Apple Watch Series 3 or later with watchOS 7.2 to access Apple Fitness+. Make sure that you're signed in with your correct Apple ID and that Bluetooth is enabled on all devices. For more info, see:

Devices you can use with Apple Fitness+ - Apple Support

I have the same issue. My software is up to date on both my phone and watch and my watch is a series 5. I have been using Apple Fitness+ but every time I start a workout from my phone, it asks if I want to use my watch. I say yes but it just sits there trying to connect/find the phone. Both the phone and watch are connected to Wifi (and cellular if I like) and both have bluetooth on. What can I do to fix this?

I am having the same problem. I have an iPhone 12 and the latest watch. Everything is up to date. It works fine with my Apple TV at home but cannot find my watch on my iPhone. It is really frustrating.