I'm not able to enable ECG on the health app on my iPhone.

First thing first, make sure you have updated to watchOS 5.1.2

Take a look at this for set up of the ECG feature

https://support.apple.com/en-us/HT208955

It is a very neat feature and seems to work fine on my watch. Remember it will only work on a Series 4 Watch

Thanks for asking about the new ECG feature. 

Check here for availability in your region: ECG

ECG requires the current non-beta version of iOS and watchOS.

For additional help with the ECG feature on your Apple Watch, take a look at these articles.

Heart rate notifications on your Apple Watch - Apple Support

Taking an ECG with the ECG app on Apple Watch Series 4 - Apple Support

If you have any other questions please reach out to our team here: https://apple.co/HeartFeatures. If you believe that you are experiencing a medical emergency, please dial your local emergency services.

Take care

It looks like it is set up, go to you Watch App > App Layout > Look for the ECG App

mine was all the way over to the right and hard to find the first time. Once I found it, I moved it to the left hand side.

Same. It doesn't let me set up through the Health App on iPhone.... 😖

Sounds like you've done step one.

Set up the ECG app

1. Make sure that the software on your iPhone and Apple Watch is up to date.
2. Open the Health app on your iPhone. 
3. Follow the onscreen steps. If you don't see a prompt to set up, tap Health Data > Heart > Electrocardiogram (ECG). 
4. After you set up ECG in the Health app, the ECG app appears on your Apple Watch.

by chance, are you under 22, the software will not work if you are under 22 and it is only working for United States at present.