Since you can't click the Avast Tray icon Left or Right Click, there is something wrong with your installation.
Is there an avast Desktop Icon ?
If so double click on that, it should open the Avast UI > Menu > Settings > General > Troubleshooting - scroll down to Repair APP. Allow that to complete and restart your system.o
You should be able to enter the UI again and Select Enter Passive Mode. The reason I suggest this way is your installation isn't working correctly, so I wouldn't try to get into passive mode whilst it isn't working correctly.
I don't know if an alternative quicker option might suffice, turning the Shields of for a period of time, but that couldn't be selected when you couldn't get access this if you can't use the Right Click option on the Tray Icon.