In the Galaxy Store you can not download the Samsung Health Monitor application so that, I can run in the watch3, the ecg option and blood pressure, someone maybe know what is going.
Could try 1) Check in Wearable app that your watch software is up to date , then 2) Open the Blood Pressure app on your watch and see if it refers to the Health Monitor app and then gives a link on phone ( or in the Wearable app on phone ) to download
I can't remember exactly how I did it but was something along these lines