My ECG app on my watch won’t read my finger on the crown. How can I fix this?

Did you follow all the steps in the 'Take an ECG' section of:

Taking an ECG with the ECG app on Apple Watch Series 4, Series 5, or Series 6 - Apple Support

Also note: The ECG app is currently available only in certain countries and regions. Learn where the ECG app is available.

Then I would try the steps in the 'Clean and dry the Digital Crown' section of:

How to clean your Apple Watch - Apple Support

The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

If you still need help, contact Apple Support.