Good morning Joe
Are you using the WhatsApp application oder the WhatsApp-Web version? If you're using the WhatsApp-Web it's possible that avast is using an adblocker which includes WhatsApp-Web.
In this case, you should go to your Browser Addon and define
www.web.whatsapp.com as an exception, so that the programm doesnt identify the page as an possible ad.
I hope this helps, good luck!