Step 1: Go to the person profile whom you wanna enable chat lock.
Step 2: Now click on the profile picture area and there you can see Chat Lock feature.
Step 3: Now enable 'Lock this chat with fingerprint' option.
Step 4: Now a pop-up will show off and tell us 'This chat will not be locked on your linked devices.'
Step 5: Confirm your fingerprint to lock the chat.
Step 6: You can access these locked chats by going into main WhatsApp interface and swipe down.
Step 7: That's it, You successfully enabled Chat Lock.
Note: This chat will not be locked on your linked devices.