can't upgrade a phone (have 10 lines) because website says I am over the total number of lines.

Hi, I currently have 10 lines on my xfinity mobile account.  All the phones are fully paid off.  When I try to upgrade one of the phones, I can't seem to add it to my cart when shopping on the xfinity website.  I get a message. " "You’re over the total number of lines available to you. A line is needed for each device added to your plan. You’ll need to remove one or more devices from your cart to continue."

I think the system thinks I'm trying to add an 11th phone line, but I'm actually upgrading an existing line.  When shopping for the phone, I selected  that I am upgrading an existing line,  and then I see the message to "Select which device you would like to upgrade", and picked an existing line with a paid off phone..  It then gives the errors message and  won’t let me add the phone to the cart.  I tried chatting with an agent who said they don't see that error on their system and that I could try purchasing a phone with agent assistance.  

I see that others have had this problem on the xfinity forums, and an agent needed to make some configuration changes to their account to resolve it.

I was wondering if my account can be fixed so that I could add a phone to upgrade an existing line on the website?

Thanks so much.