I found a solution posted by someone else on another forum and copied it down below. This requires unpairing the watch AND a reset all settings on the phone itself. I have not experienced this issue, but others have.
Got a fix from Apple tonight - I would call it a work around until Apple gets it into an update. Bottom line the issue is with the iPhone and in the settings.
Step 1: Unpair the watch
Step 2: Settings - General - Reset - "Reset All Settings" Going to have to re-setup somethings on the phone but its better than a full restore.
Step 3: Pair the watch - either as new or restore from backup. I restored from a backup.
Step 4: Open the Watch App on the iPhone.
Step 4a: Go to "Heart" in the list of Watch Apps
Step 4b: Select "Set up ECG App on Watch"
a shortened version implies you don't need to unpair the watch first.
Settings - General - Reset - Reset All Settings (on your iPhone)
I restarted my iPhone and did a force restart on my Apple Watch, then went on to the Health App on my iPhone and I had the option to “Set Up” the ECG app again!