TL;DR - try to follow Jakub's guide at youtube -
https://www.youtube.com/watch?v=XrkiUFtgdYE&feature=youtu.beLong story :
Apple introduced new security feature "User-Approved Kernel Extension Loading" in High Sierra release of macOSX (details at
https://developer.apple.com/library/archive/technotes/tn2459/_index.html). This feature requires user to manually allow any new kernel extension (aka kext). Avast antivirus is using these extensions for enabling real time protection of your mac. The real time protection covers any new files downloaded/copied or started from your hard drive (File shield), it checks any new emails coming to your Mail client (Mail shield) and verifies that any website content and URLs you've visited are safe (Web shield).
You'd need to go to Preferences > Security&Privacy and click ALLOW button. Unfortunately, we have noticed that there were some usability issues in particular High Sierra builds. According to Apple and our experience the issues that "user clicks the ALLOW button but nothing happens" significantly improved in macosx Mojave release. In some other cases the mentioned youtube guide helps. But neither of these workarounds works in 100%. I'd love to help you more but there is no way how we could workaround such bugs within the macosx.
There is one more trap in the "User-Approved Kernel Extension Loading" feature - the ALLOW button disappears after 30 minutes. Restarting of your macos should fix it -> restart the timer.