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.