Follow these steps to enable Whatsapp Fingerprint option :

Step 1 : Open WhatsApp and Go to Account.

Step 2 : And go to Privacy

Step 3 : Then Choose fingerprint lock

Step 4 : Then Enable Fingerprint lock and Confirm fingerprint

Step 5 : Then Choose immediately option

Step 6 : Now Fingerprint lock was enabled on your device..

Step 7 : Then close WhatsApp and open it again it will ask fingerprint to open WhatsApp.