unlock - iPad Air 2 IMEI ID not recognized

Attempting to unlock my iPad Air 2, which I purchased from an AT&T Wireless store. This device has been on my AT&T Wireless account for 27 months. I even received a notification from AT&T acknowledging the device was eligible to be unlocked.

When I input the IMEI ID on the unlock website, I receive the following error message:

"The IMEI you entered does not appear to be an AT&T wireless phone or tablet."