Att hasn’t sold them in a long time, when they did, they were for prepaid. This means ATT will not unlock unless it (the phone) is used on prepaid for 6 months of paid service.
Sell the phone to someone coming to the US who will use it.
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We would need more information. Why was the unlock denied? If the previous owner still owed money on the phone or it was lost or stolen, then you'll need to either have the previous owner resolve it or get a refund because AT&T won't unlock it for you.