IPhone 6s Pass code keyboard

The passcode unlocks the phone’s screen. If it is asking for a 4 or 6 digit code the phone was not properly reset by the previous owner. You can try resetting the phone yourself:

See this Apple support link→If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Or watch this companion video ⬇️

After doing that you will need the Apple ID and password that were used to enable Activation Lock. Without that information the phone is useless. Return the phone or ask the seller to properly reset it.

See→The All Too Common SAD Reality of Buying a Used iPad/iPhone

See point #3 on section "Set up a passcode" in this support article:--> Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio