Trying to unlock country lock

I have been trying to unlock my phone through AT&T unlock device online but it keeps telling me that I have to pay off the device first. Error message: 

Looks like you haven't paid off your device yet. Follow these steps to pay off your contract. Then, wait 24 hours before you submit a new request.

I came back to my country from the US so i tried to cancel at&t prepaid plan but i can't receive 8 digit number for cancelling because i'm outside of the US. I applied new prepaid plan though i won't use. However, error message is still occurring.

What should I do to get my phone unlocked?