Why is it that my cardio fitness has no data?

It says in the link I posted that VO2Max can only be measured by doing an outdoor run,walk or hiking workout.

Quote;

How Apple Watch estimates your cardio fitness levels

Apple Watch Series 3 or later can record an estimate of your VO2 max using the heart and motion sensors during an outdoor walk, run or hike. It can also estimate your VO2 max if you start an outdoor walk, run or hike in the Workout app. Apple Watch supports a VO2 max range of 14-60 mL/kg/min that is validated for users aged 20 or over.

How Apple Watch estimates your cardio fitness levels

Apple Watch Series 3 or later can record an estimate of your VO2 max using the heart and motion sensors during an outdoor walk, run or hike. It can also estimate your VO2 max if you start an outdoor walk, run or hike in the Workout app. Apple Watch supports a VO2 max range of 14-60 mL/kg/min that is validated for users aged 20 or over.

To estimate your cardio fitness level, your Apple Watch also takes into account your:

• Age
• Sex
• Weight
• Height
• Medications that may affect your heart rate

It can take at least 24 hours wearing your Apple Watch, followed by several workouts* and passive measurements with your Apple Watch before you receive an initial estimate.

For some reason the cardio fitness VO2Max relies on GPS. Thus, if you workout is on a treadmill or elliptical it wont measure your workout. Go outside for as walk or run and it will measure and record your cardio fitness,

Apple needs to fix this so it does not rely only on GPS.

It won't be in data and sources until there is some data recorded there.

Here is the full page about cardio fitness;

Set up Cardio Fitness Levels – Apple Support