Hello __Spartan__,
Apple Support Communities is a great place for help with the VO2 max estimate, and we’re glad you were directed here from the Set up Cardio Fitness Levels article.
You mentioned using a chest strap to measure your heart rate. If you don’t use that strap, does the VO2 max estimate update? If so, you may want to contact the strap manufacturer for additional support. If the issue persists without the strap, make sure you’re running the latest iOS and watchOS. You can find the steps for updating in the following articles.
Update your iPhone, iPad, or iPod touch
Update your Apple Watch
If you’re already running the latest iOS and watchOS, the next step is to restart your devices. You can find the steps for restarting in the below articles.
Restart your iPhone
Restart your Apple Watch
If you continue to have issues after those steps, it is recommended to contact Apple. Their contact details can be found via the Get Support website.
Take care!