Did you already try to force restart your iPhone and Apple Watch?
Force restart iPhone - Apple Support
To force restart your watch, press and hold the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again.
If this does not work, and to rule out a software issue, erase the watch and pair it again, setting it up as a new device. You will only lose your created Watch faces and may have to activate the cards in your Wallet app again.
Then check the cardio fitness data in the Health App again, after recording an Outdoor Walk or Outdoor Run.
Unpair and erase your Apple Watch - Apple Support