Digital Crown not registering ecg

Restart your watch.

Assuming you are now on WatchOS 9.5 (it was released yesterday, so if you haven't updated, update your watch).

Press and hold the elongated power button until the SOS screen appears. Tap the little power icon in the upper right hand corner. Swipe to power off. Wait a minute, then press and hold the power button again, holding until you see the Apple Logo, then let go. See if ECG registers after you sign back in to your watch.

Do you by chance live in a country where ECG has not yet been approved by your government?

I don't have more suggestions. I'd schedule an appointment at your Apple Store Genius Bar and let Apple test the watch directly.