Activation lock on my iPhone 6s

You need to return the phone to the seller. That could be a stolen phone and you won't get help from Apple to remove the activation lock. That is the purpose of activation lock.

ASC - Late 2012 Mac mini, macOS 10.14.5; Watch, series 3 cellular, watchOS 5.2.1; iPad Air 2 & iPhone 8+, iOS 12.3.2; TV 4th Gen, tvOS 12.3;  HomePods, audioOS 12.3 and Airport Express w/ Airplay 2

If you can contact the seller, you will need to have them log out of iCloud or go to iCloud.com/Find My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless. Apple will not unlock it unless you have a copy of the original purchase receipt with the serial number on it.

Activation Lock - Turn off Find My iPhone