I can’t get my ECG in my Apple Watch 4 that was bought in USA?

Taking an ECG with the ECG app on Apple Watch Series 4 or later - https://support.apple.com/HT208955 - "Make sure that the ECG app is available in your country or region and that you're in the country or region where your device was purchased." So unless it is supported both in the country of purchase and where you reside, it won't work.

Has the ECG app been approved in your country?.

watchOS - Feature Availability - Apple

EZittan wrote:

Why I can’t get my ECG in my Apple Watch 4 that was bought in USA?

[Re-Titled by Moderator]

That would depend on where you are currently located. If you are not in a location where the ECG has been approved, it doesn't matter where you purchased the watch, it won't work.