Open WhatsApp
Open the chat you want to lock
Tap the contact or group name at the top
Tap Chat Lock
Turn on Chat Lock
Authenticate with fingerprint, Face ID, or device passcode
The chat will move to the Locked Chats folder
To view it, go to the Chats screen and swipe down
Open Locked Chats and authenticate again
To unlock the chat, open it and turn off Chat Lock in the chat info screen