User Approved Kernel Extension Loading. To improve security on the Mac, kernel extensions installed with or after the installation of macOS High Sierra require user consent in order to load. This is known as User Approved Kernel Extension Loading. Any user can approve a kernel extension, even if they don’t have administrator privileges. Change Extensions preferences on Mac. On your Mac, use Extensions System Preferences to enable and disable Apple and third-party Mac extensions, and select extensions to include in the Share menu. Extensions, such as Markup, add extra.

On another note you really don't need Avast. There are NO known viruses for Macs. All Avast does is lighten your wallet and adversely affect the system performance. You should remove it according to the developer's instruction: Uninstalling Avast Security from Mac | Official Avast Support

Additionally there is absolutely no need for any A/V, 'cleaning', 'optimizing'or 'speedup' apps for the Mac. They are known to do more harm and no good.

This user tip is an excellent treatise on how to protect your Mac: Effective defenses against malware and other threats

Avast For Mac High Sierra System Extension Blocked Download

May 30, 2018 1:03 PM