Why is my Apple Watch not displaying Vo2 Max data for cardio fitness?

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

This is from the information on your iPhone on the Activity app.

>>Measuring VO max requires a physical test and special equipment. You can also get an estimated VO, max with heart rate and motion data from a fitness tracker. Apple Watch can record an estimated VO, max when you do a brisk walk, hike, or run outdoors.

VO2 max is validated for users 20 and older.

Most people can improve their VO, max with more intense and more frequent cardiovascular exercise. Certain conditions or medications that limit your heart rate may cause an overestimation of your VO, max. VO2 max is not validated for pregnant users. You can indicate you're taking certain medications or add a current pregnancy in Health Details.<<

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