Workout heart rate chart became unavailable

Three choices immediately come to mind:

1. The watch heart rate sensor is defective- it happens.
2. You are not wearing it properly.
3. You are wearing the watch over a tattoo or possibly a significant scar.

Other possibilities include not having fitness and heart rate tracking enabled under privacy but that seldom seems to be the case in the real world (just obvious things to check).

https://discussions.apple.com/thread/8099871

Thanks

Try a forced reset;

To force reset your Apple Watch, press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.