Unlock Phone

Submit your request as a non customer. Where it asks if you are a customer, select no. 

Yes, there. All you need is the phone’s imei.

Well it’s right there in black and white. You can’t unlock it 

Well yes, but now I'm the owner of this device right?

Not according to AT&T. Someone else owns it you just have possession of it 

Thanks!
What can you recommend me now?

Sell the phone there is nothing you can do.  

@Tudor  Only the original owner can fix the issue. You are not the original owner.