Chosen Solution

i don’t know the apple id of the previous iphone 6 user

You can’t bypass the Apple ID. Don’t fall for the scammers out there they can’t either. All they do is enable the emergency phone mode so you have a dumb smart phone! Cheaper to buy a flip-phone in that case. Return the phone and get your money back as its sounds like a stolen phone! Otherwise you’ll need to get to owner to release it from is Apple ID View and remove your associated devices in iTunes