Well it's advertised as you can upgrade one line and get one free if that's a new line of service.

You do fulfill the new line of service part, but not the one line must be eligible to sign a new device payments purchase; you must be of both

The thing is to get it you must sign a new device payments agreement, and you can't do that until your current one is fulfilled.

And I would be compassionate, but you must think of this logically. You are currently already on an agreement on your phone and have not yet paid Verizon back for it, therefore while your still in the process of paying that one off, you can't get another agreement

You can get a new line activated for 20$/mo (on old shared plans) on top of the one you were planning, and get that one with a free Pixel 3.