My bill is not due till the 21st.
Since I have not fufilled my Next commitment yet AT&T's website says this:
To upgrade now, you'll just need to pay the remaining balance of your initial 12 or 18 installments and trade in your qualifying device. Or, upgrade without a trade-in by paying off all the remaining installment payments.
I select "Pay the remaining balance" and put in my credit card number and submit my order then I get the error posted in my original post.