How to fix vo2 not working after iOS 26.1 update?

Which Apple Watch do you use?

Did you also try to reset the iPhone to factory settings, to rule out, that the iOS software caused the issue?

To rule out defective data from the backup, set it up as a new device and record an Outdoor Run or Outdoor Walk.

Restore your iPhone, iPad, or iPod to factory settings using a computer - Apple Support

If VO2 has been recorded in the Health app again, restore from the latest backup and test again.

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.

In case this does not work, unpairing and setting the watch up as a new device, without using the latest backup would be the way to rule out a software issue, caused by defective data from the backup.

Unpair and erase your Apple Watch - Apple Support

You need quite a few workouts to populate the data.

Are you doing regular outdoor walk or run workouts? No other workout will generate VO2