You can use the Chat Lock feature to password- or biometrically-protect your WhatsApp conversations.
Go to Settings > Account > Privacy > conversation Lock to enable conversation lock.
To lock your chats, you can use a PIN, face unlock, or fingerprint.
The first time you open a locked WhatsApp chat after enabling chat lock, you will be required to enter your PIN, face unlock, or fingerprint.
Chat lock can assist in preserving your privacy by limiting who can view your chats.
By entering your phone number and selecting "Forgot PIN?," you can reset your PIN if you lose it.