I'm not sure this will help, but this information comes from:
https://www.cultofmac.com/610268/vo2-max-apple-watch-aerobic-fitness/
Apple Watch may log your VO2 max, but you can’t currently check it on the watch itself. To see your stats, you must use the Health app on your iPhone. Go to Health Data > Activity and scroll down to find it. If you have readings for the current month, they will be highlighted in a big orange box. If you have no VO2 max data, it will be languishing at the bottom of the list, under No Recorded Data.
If you can’t find any VO2 max readings in the Health app, that is probably because you have not yet logged the right kind of workouts with your watch. Only two workout types can produce an estimated VO2 max on Apple Watch: Outdoor Walk and Outdoor Run.
VO2 max relates to maximal exertion. So you’ll need to log a pretty intensive workout to generate a reading. And you’ll need to sustain it for at least 20 minutes.
The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:
How to restart your Apple Watch - Apple Support
Restart your iPhone - Apple Support
If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support