In my limited experience, (iPhone 6s and Watch Series 6) you cannot activate a watch on Verizon unless the phone with which you want to activate and use the watch has already had the My Verizon app configured appropriately. The activation process will appear to work, but will fail at the Verizon log in, even if you have a log in and password registered with your Verizon account outside of the My Verizon app. I am a very long time Verizon customer (long before the My Verizon app) and simply never bothered to use the My Verizon app to manage my account. Also, I could not use my existing log in credentials from within the My Verizon app, but had to create new credentials within the My Verizon app. THEN, the activation process worked smoothly.
I hope that this prevents someone else the head banging I just put myself through!
I hope that this prevents someone else the head banging I just put myself through!