How to Lock WhatsApp Chat on an Android device
Open your WhatsApp and go to the Settings.
Choose the Account option.
Now go to Privacy.
Tap on privacy and scroll down to Fingerprint lock.
Enable the Unlock with fingerprint option.
Now Confirm fingerprint – Touch the fingerprint sensor
Fingerprint lock will give you options to choose the time duration to lock Whatsapp. Select your preferred option.
Now your WhatsApp Chat is locked.