Try this;
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.
Or this;
1.turn airplane mode on the watch to ON wait a few seconds and turn it Off again,this has worked for some people in the past.
2.force quit the Activity app and re-launch it.
make sure you are in the app. Then, you have to press and hold the Side button to bring up the power down menu. Then, release the side button. After that, press and hold the Digital Crown. The app will be killed, and you will go to the Home screen.Then relaunch the activity app again.
3.Close the Activity app on the iPhone
On your watch: go the Home screen
(single press of the Digital Crown) and tap on
the Activity app icon (coloured rings).
Press firmly on the screen > tap on Change
your Move Goal > change the goal.
You can move the goal back once it's working again.
4.On your iPhone, go to: Settings > Privacy > Motion & Fitness - check that both Fitness Tracking and Health are enabled.
On your iPhone, in the Watch app, go to: My Watch > Privacy > check that both Fitness Tracking and Heart Rate are enabled.