try s secure. but if you think that is not comfort for your need, then install app lock from play store which the link is below. if you want to lock your whatapp alone then there is a inbuilt fingerprint lock in it itself (available only on fingerprint option available devices only) https://play.google.com/store/apps/details?id=com.domobile.applockwatcher
bro download from playstore as "S secure" is not much secure as anyone can uninstall it and it is not asking any pin or passwords before uninstalling but in case of app lock shouldn't be uninstall without asking pin/password so u can check this out by this app lock you can lock settings, notifications, and even recent tabs.
