Trying to upgrade phone

I have been trying to upgrade my phone for the last three years and I get this message: You’re over the number of lines you can have on device payment plans. For one or more lines, you’ll need to pay in full or bring your own devices to continue. Or you can reduce the number of devices in cart.

I have been a xfinity member for 9 years and have purchased 3 phones on payments.  My credit is not bad.  I am the only person on this account, just one line.  Why does this keep happening?? This has been happening to other people as well.  I tried contacting customer service and they said thre was nothing they can do, as they cannot overide the system. I asked for a manager, no one called me back.