why can't I use ECG?

Without additional information we have no idea! Please see the requirements (and if you meet the requirements how to setup ECG) in Taking an ECG with the ECG app on Apple Watch Series 4 - Apple Support

I just noticed your user name has a clue about your location, which suggests ECG is not currently available in your location. https://www.apple.com/watchos/feature-availability/#apps-ecg

I'm in Australia and I have the same issue. Apparently in my region, Australia, our governing medical body haven't approved the Apple watch S4 for use as an ECG device. It's a real shame.