Upgrade at Walmart failed with error

Tried to get my wife's iphone upgraded last week at Walmart. We are all within a family plan while a friend of us is the account holder. My wife had been added as authorized user to the family plan, and she did upgrade her phone successfully 2 years ago at Sam's Club.

This time, we failed when trying to upgrade at Walmart. All the information we provided seem correct, since we can go through all the steps all the way to the end of the process. After scanning the bar code of the iphone box, the order was submitted. However, while it supposed to display a "Success" screen, it returned with an error massage saying "...the order can not be processed...and a letter will be sent to account holder to provide more information...". And on the screen, it showed the order of upgrading had been cancelled.

Called customer support, but no answer to why this happened. Also went to an ATT local store to see if they know why it happened, and still no luck. Nobody knows why the order got cancelled while all previous steps were successful all the way through.

Any answers?