Problems with AT&T Consumer Device Unlock Portal

Hi there! I'm from Mexico, I have a question, last week, my father bought me a Nexus 6 with a sticker that says "Unlocked" from an AT&T store, but when I wanted to put my sim from a local carrier, the phone requests me a Pin code. I assumed that the phone was not unlocked. So I found that portal, and I put my IMEI to request an unlock. But the site says:

Error: We found no record of the IMEI Number you entered. Check your device's 15-digit IMEI Number by dialing *#06# (error Code.ULP 2003)

What can I do?

Sorry, my english isn't good. Thanks