I did every step from this article including removing my Apple ID and iCloud data from phone and iPad.
https://www.myhealthyapple.com/ipad-not-finding-your-apple-watch-for-apple-fitness-and-the-fitness-app/
I updated to the latest OS on watch, phone, and iPad. I talked with Apple support for an hour before they accidentally hung up on me. Nothing made any difference. My iPad could not find my watch.
THE FIX that finally worked, at least for me, was to UNPAIR my watch. Unpairing completely erased all my data and I had to go through the whole pairing process like a new watch. This took about 30 minutes but afterwards my iPad found my watch when starting a fitness + workout just like it did before this problem. I wish I would have tried this first. It really was easy since all my data synced back to the watch after pairing again.
Apple Watch 6
iPhone 11
iPad Pro (12.9”) 3rd Gen