Why can't I add a new line/device?

I have an iPhone8 plus and the Go Unlimited plan.  I tried to purchase a Series 5 Apple Watch, through the website, but, at the last step of the process the activation and number share options were grayed out.  I tried again through the My Verizon app on the pone, and got the error message "You have a connected device in your cart which is not number shared. Please edit the device to number share or shop compatible iPhones."

Why is my phone showing up as not compatible with adding a new line/device? Is it a feature of my plan? (And if so, why can't Verizon make that information apparent)?

Re: Why can't I add a new line/device?
Customer Service Rep

Having a watch to use with your plan and phone can be awesome HadrianV. I'm sorry you're having issues with adding it as a new line and completing the purchase. Will you be sharing this watch line with your phone line? It is required when adding a watch line to your account to designate a phone line on that account as the phone that will be sharing services. You will also need to change the plan on your mobile line to a current plan we offer because you're adding a connected device line that will be sharing services with the phone line. Here is a link below with details about the Go-Unlimited plan and how it works with new lines because its a retired plan. If you continue to have issues adding the watch line, please call our sales team at 1-800-225-5499. They can help with the process and make sure you have the right plan for both lines.