OK–so I decided to Google and see what I could come up with specific to the 6MW test and Apple Watch. Apparently the Six-Minute Walk number given by the Apple Watch is an estimate of what your performance/distance on the formal six-minute walk test, derived from your other physical activity over a 1 week period.
https://discussions.apple.com/thread/252008008
Specifically, the screen shot description there begins: “This is an estimate of how far you can walk on flat ground in six minutes based on a week of your motion and workout data.” So if there’s no result, it may be because there isn’t enough data in the watch yet to make an estimate. It does not seem that the watch actually measures your distance over six minutes and gives you an actual result, but that it derives an estimated result from other data.
Dave