Updating & scanning with non root user via GUI and CLI works no problems. You may need to reinstall your avast and see it that helps, note I installed my copy via RPMs and using YaST.
(Assuming this is your first time using Avast in Linux, if its not then you can skip this part) In order for it to work you first need to run the GUI version of it, if running KDE you can do this easily by hitting Alt + F2 and typing in avast, or you can look for it in your menu. After you ran the GUI for the first time and entered in your registration you can choose to keep using the GUI or use the CLI.
To use the CLI just type avast and tell it the options and where to scan, it defaults to scanning the current users home.