Have a look here an scroll down to 'allow full disk access':
https://support.avast.com/en-us/arti...rmissions/#mac
A couple of MacOS versions ago, Apple tightended up the system in that you had to specifically allow full disk access for specific applications that needed this (as well as other permissions), somehow your MacBook Air ran an update that either reset this or it updated to a newer version that no longer allows Avast to run without this permission. You have not been hacked. I think your iMac is actually the less secure one as an older MacOS version may have some security issues that have been resolved in newer versions. The difference between the two Macs is simply in the MacOS versions that you're running.