i have noticed when you first pair an S6 watch it shows up (ensure you enable it in health app on phone under > Browse > Heart).
it disappears after that, this might be minutes, or up to a day.
I contacted Apple support, they mentioned they are aware of an issue, asked me to update both iOS and watchos, 14.1 was available so I did, watch is already on latest (7.0.2 at the time of writing). This made no difference to ECG app situation.
I then unpaired and repaired the watch, which brought back the ECG app, it stayed for about a day and then disappeared again. As per Apple supports advice I reset all settings on the iPhone next, to no avail.
For the moment I do not know of a solution.
Several posts suggest to look ECG app on AppStore, at least I got one cannot find it there.
iPhone 11 Pro Max ios14.1
Watch S6 44 Cellular 7.0.2
ECG supported in the region