Apple watch series6 OS 11.6.1 isn’t uploading VO2 max measurements to Iphone IOS 18.6.2

VO2 max is only estimated by the Health App if you record an Outdoor Walk, Outdoor Run or Outdoor Hiking, using the Apple Workout App. And, it will take more than one workout, until a VO2max can be estimated by the Health App:

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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.

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copied from: Track your cardio fitness levels - Apple Support

and if you want other workouts to calculate VO2 max such as cycling then suggest it

https://apple.com/feedback