Q: what to do when you buy a second hand iphone but stuck in previuous owner icloud iccount

Very sorry, but the iPhone is useless to you until the prior owner removes it from their could account.

Return it for a refund.

If you can't contact the previous owner, there is nothing you can do:

Turn off Find My iPhone Activation Lock - Apple Support

Apple will not help you.  Try to return the device and get your money refunded or dispose of it properly.  The device is now a brick.