Unlock AT&T Phone

I recently finished paying off my phone from AT&T and would like to unlock it now. However, every time I enter the IMEI I get the following message "Looks like the IMEI number you entered doesn't match an AT&T device. Try entering it again. Dial *#06# on your mobile phone to make sure you have the right IMEI"

I just don't know what to do. I've spoken with an online chat representative and still can't solve the problem. Like I said, I finished paying off the phone so I don't understand how it could not be an AT&T device? It's on an active phone plan with AT&T also. I've checked the unlock phone eligibility requirements and I've met them all to the best of my knowledge.

Any help would be greatly appreciated,
Thank you.