Device Unlock error: "Your data cannot be validated by the system."
I am trying to unlock my old iPhone, but I don't have an account with AT&T anymore. I bought the iPhone in December 2013 on a 2 year contract, but than I cancelled my personal contract (and paid the remainder) in June 2014 when I switched to AT&T business account through my work. I didn't have to unlock my phone since I was still using AT&T, but now that I bought a new version I am trying to unlock the old iPhone so I can sell it for more. The problem is that when I enter the IMEI and fill out the rest of it on this page I get an error back saying that "Your data cannot be validated by the system."
How can I get pass this error?