ECG App missing on Apple watch

One of the common reasons is location.

Watch only offers ECG support in a country or region where the ECG feature is permissible.

Here is the list of countries and regions: watchOS - Feature Availability - Apple

Another reason is a Watch user that’s under 22 years of age.

Details: Take an ECG with the ECG app on Apple Watch - Apple Support

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.