To my knowledge, samsung does not give an app lock outside of applications that request for your phone security for different actions in the app (ex. Logging into a banking app, Opening a paying service like Google Pay and Samsung Wallet, logins for different apps, etc) and Secure Folder, which is a separate pin/pattern/password that you put in to access it. If you are trying to go into an app with a lock on it, it may be another app that is locking the app via its own software. You should be able to find these apps under Settings>Apps. However, if the app provides it's own lock, you will need the pin for that specific application. Usually you can tell the difference between the two.
