At&t Unlock error - imei not identified

I need to activate the iphone that says not to be in the bank of att when I type the imei in the site of the unlocking, I have already activated it in a store but when I put another operator's chip it returns to the activation screen. Someone help?

error: "The IMEI you entered does not appear to be an AT & T mobile phone or tablet. Re-enter the 15-digit IMEI number for your AT & T mobile phone or tablet. You can get the IMEI number from your mobile phone by dialing * # 06 #."