Feel free to share with your more ignorant users to educate them on the reality of the threat landscape out there. Here's a small sampling of some of the more recent Mac threats if you are interested. That being said, there have been plenty of instances of Mac viruses written, many that we have documented and that we detect in our products. If you are going to write a virus, you might as well write it for the OS that has 87% market share since you'll get a lot more infections out there, more botted machines, more stolen credentials, etc. In other words, the bad guys (that's my technical term for all the threat actors involved in the malware/virus/spyware supply chain) spend their time focusing on the market share leader (Windows) so as to have the biggest ROI.
To add the exception open Avast Security Dashboard, click on the settings cog in the top right, then click on. The reality is that Macs aren't any more secure than Windows generally speaking, the reason there aren't as many Mac viruses is a market share issue. A better way to handle a security suite that's blocking software you trust enough to run would be to add an exception into Avast Security, so that it doesn't block the previously mentioned software.