First of all Whatsapp icon is only appears on mobile devices if you want to show it on all devices like PC, Laptop, Tablets, and Mobiles then simply go to the Dashboard >> Porto >> Theme Options >> Skin >> Custom CSS and then paste this code
.share-whatsapp{display:block !important;}
in Custom CSS section as showing below:
Now to fix the Whatsapp icon issue on header, when user click on Whatsapp icon it will let user to chat on Whatsapp. Simply go to your site cPanel and open the layout.php file under wp-content/themes/porto/inc/functions now when the file opens, scroll down to the page at line number 2141 there you see
Anchor
tag, simply copy this code