Try this first;
Follow these steps to set up the ECG app:
Open the Health app on your iPhone.
Follow the onscreen steps. If you don't see a prompt to set up, tap the Browse tab, then tap Heart > Electrocardiograms (ECG) > Set Up ECG App.
After you complete set up, open the ECG app to take an ECG.
If you still don’t see the app on your Apple Watch, you can search the App Store on your Apple Watch for ECG and download it.
Plan B;
On your phone go to: Settings/General/Reset/Reset All Settings.
This will erase your settings (without losing any data) and will allow you to re-setup the health app and re-enable the ecg app.
Be aware though that this will delete any cards setup in apple pay.