iPhone6 IMEI mismatch issue for unlock

I'm trying to unlock my iPhone 6 device with its IMEI, but the website gives me an error message saying "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'm 100% sure that I'm using the right IMEI number, but can't resolve this issue..

Thank you for your attention to this matter in advance!