I am unable to use blood oxygen feature on my apple watch 8 gps
Apple Watch Series 8, watchOS 11
They are talking about Blood Oxygen not VO2 max.
It could be a country limitation, https://www.apple.com/uk/watchos/feature-availability/
but if not then
Apple Watch basic diagnostics - Apple Community
Did this work in the past? And, it will take more than one workout, until a VO2max can be estimated by the Health App:
...
It can take at least 24 hours of wearing your Apple Watch, followed by several Outdoor Walk, Outdoor Run, or Hiking workouts* and passive measurements with your Apple Watch before you receive an initial estimate.
If the Health app detects that your previous cardio fitness estimates could be more accurate, you'll have the option to update your data. In Cardio Fitness, tap Update under Update Your Data. After you update, the Cardio Fitness Data Updated notification shows you the results of the update and how your estimates changed.
...
copied from: Track your cardio fitness levels - Apple Support
Spo2 not working on my Apple Watch 8
