I turned off the heart rate monitor on my watch via the settings | privacy | The heart rate alternates because I didn't want the watch to keep a constant record of my HR device and extend the battery life.
However, when I exercise, I will check my HR unit and use a Polar H7 strap for accuracy, but no matter what I did, the training app would not see HR, nor any other exercise apps like Strava, even though H7 was the right pairing.
Then it turns out that the switching of the HR privacy switch has the "bonus" side effect of disabling AL HR tracking in ALL apps. Which means if I want to use the heartbeat, I have to disable the privacy setting before each workout OR use my iPhone to run Strava (but not the apple training), which seems to defeat the purpose of having the apple clock initially. [1