I have tried both swiftunlocks and unlockfusion which are the ones I normally use. However, they failed on me this time. Any cheap working sites? 
I thing if your iPhone is in contract AT&T company not allow you to Unlock your device. Only out of contract they allow to Unlock. I know this, is possible I to be wrong, but I this know.
Here you go, an entire thread on it.
https://forums.macrumors.com/thread...evice-still-in-contract-for-free-ymmv.1837666