Activation Lock Asking for Apple ID That's Never Been Used on this Phone
You will need Proof of Purchase like a printed Invoice showing SN.  You call Apple ask for Senior Advisor who will have you upload Proof and then it takes 48 Business Hours to unlock.  Then put iPhone in DFU mode and restore on iTunes.  Don't be fooled by any of the uTube scams claiming an unlock.  I have one (of 3) that relocked 3 times but there is hope they may have fixed something after they were hacked.  All of my iPhone 6S had AppleCare +.  If you have an Apple Store near go with Proof and see Manager.  They know about the problem but won't talk about it and have exchanged some iPhones for ones with good SN.  It's also happen to a few iPhone 7s.  There is a Macrumors thread 15 pages long of people with the same problem.  Good Luck.