The IMEI you entered does not appear to be an AT&T wireless phone or tablet.
I am an AT&T gophone user. I was trying to unlock my Samsung Galaxy S8 which I preordered from the Samsung official website. It is an AT&T version. I was trying to unlock it because I will go to China during the summer, and I want to use a Chinese SIM card during that period. (And I will still use AT&T in the US)
However, when I input the IMEI number of my S8, the website shows "The IMEI you entered does not appear to be an AT&T wireless phone or tablet. Please re-enter the correct 15-digit IMEI number for your AT&T wireless phone or tablet. You can obtain the IMEI number from the wireless phone by dialing *#06#". I've tried it several times, and I'm sure I didn't input the wrong IMEI number.