How to disable Activation Lock in my iPhone 6s

As you bought a used phone, if it is activation locked you will never be able to use it. Apple will only remove activation lock for the purchaser of a new phone purchased from Apple or an Apple authorized reseller. They will never removed it on a resold phone.

And this is a PUBLIC user-to-user forum, not a good place to post personal information.

The support article you came here from should provide you with the steps needed to try to resolve this issue. Have you tried them all yet? If so, and they didn't help, I suggest that you contact Apple Support directly. To do so, simply select the "Get Support" link in the upper right-hand side of this forum's web page.

Your iPhone is likely previously stolen. Get a refund if possible.

How to remove Activation Lock - Apple Support