I have a new Apple Watch 6 series, and after 36 hours and one recorded workout I still cannot see any data in the cardio fitness section of the health app. I am getting all other info on the Health App just fine. Not seeing a set up tab in Health App

This is the correct answer. Albeit I do not agree with only using outdoor workouts for walk, run, hike as a proper metric to determine Cardio Fitness. I think there needs to be a bit more development to include indoor treadmill workouts and potentially HIIT style workouts, etc. I can appreciate that this is a 'best-estimate' calculation and have no idea as to what actual science and math is needed to make these estimates. Looking forward to seeing the apps grow over time.

Not until your watch has recorded some vo2max data;

Maximum heart rate multiplied by .70 = 70% of your maximum heart rate and it is the minimum number of heartbeats per minute that the watch needs before it will track Vo2Max and record a reading.

This hearbeat measurement must be kept up for at least 20 mins and must be constant.

Apple quotes the figure to measure Vo2Max from 70% up to 85% of your maximum heart rate so I used the lower measurement as a bare minimum in which to get a reading.

Also the watch needs a fair bit of previous workout data before it will record a VO2max reading,I've never been able to find out how much data that should be but it seems that the watch needs this data as some sort of comparison or calibration data.

Only three workout types can produce an estimated Vo2max on Apple Watch: Outdoor Walk,Outdoor Run and Hiking.

Cardio Fitness is measured during an outdoor walk, run, or hike.

Set up Cardio Fitness Levels - Apple Support