No ECG

The ECG app is availble on Apple Series 4 and newer and it must be prchesd from a supported country. Check to see if you are in a supported country here: https://www.apple.com/watchos/feature-availability/#apps-ecg

You also must be at least 22 years older to use the app. For more info on the ECG app, see here: https://support.apple.com/en-us/HT208955

It is a part of the Health App (if you are in a country where this feature is authorized).