Country for ECG on Apple Watch 4

According to this

Apple Watch ECG countries

https://www.apple.com/watchos/feature-availability/#apps-ecg

Croatia does have support for ECG.

So the question is, did you buy your Apple Watch and your iPhone in Croatia, and was your Apple ID created in Croatia? Apple uses several factors to make sure is not enabling ECG in countries where it was not approved. It is not uncommon for someone to buy one of the devices in another country while traveling, or relocate to a different country, and have their devices pointing back to the country where the device was obtained, or the Apple ID account was created.

Unfortunately only Apple knows the answer.

Unfortunately you needed buy the European model. As the USA one is regional set

You don’t. You can post feedback. They will read it.