Upgrade issues

When I go to upgrade my phone on one of the specials you have, the 'value' of my device changes. Currently, I am supposed to be able to get a Galaxy S24+ for $0.00/mnth with qualified trade in. When I use my current phone, a Galaxy Note 20 5G in excellent condition, it says my phone would be worth $1000 for the special and my monthly would be $0.00/mnth. But when I get to the cart, it switches and says my phone is only worth $98 and I'll have to pay $27.78/mnth. What's going on and why am I not getting the special? My account is current and my phone is paid off.