"The IMEI you entered is not eligible for a trade-in offer" - why not?

As the title says, I'm trying to do a trade but getting rejected. The phone I have is a Pixel 6, purchased directly from Xfinity Mobile and now fully paid off. It wasn't even part of a promo, just a regular full price phone. When I try to purchase a new phone and use the Pixel 6 as a trade-in, I get the message saying IMEI is not eligible for trade-in offer. 

I did a chat but they claim to have no idea and suggest I visit an Xfinity store. That's a multi-hour trip for me so I am trying to get it figured out here. Can anyone help? Again, there's no reason this phone, purchased direct from Xfinity and fully paid off, should not work as a trade-in.