My apple watch 6 is my first ever apple watch, bought it on Nov. V02max never worked since the beginning even though I’ve followed the 20 minute outdoor rule. This morning I tried a bit more intensely: Outdoor walking for 23 minutes average heart rate 150bpm with max 194bpm. And Cardio Fitness data finally shows up. My usual workout is 30 minutes outdoor walk avergae heart rate 110bpm. Will see tomorrow if any data will show up if I go back to my usual routine. I think you must reach certain heartrate for it to begin collecting data. Hope it works for you.
PS I recaliberated my watch before I started my workout this morning.