Install ECG app on watch

If you cannot find it in App Store I suggest you unpair and set up again, if necessary as a new watch

Resources:

 Unpair and erase your Apple Watch – Apple Support (UK) 

and then set up again - takes about 20 minutes

Set up your Apple Watch – Apple Support (UK) 

If necessary unpair and setup without restoring the backup

In addition to the above, you must be in a country or region where ECG is permitted:

… watchOS - Feature Availability - Apple