The lack of scan scheduling in Avast is one of the limitations of running the free version.
Some AV companies "cripple" detection, updating, and/or cleaning features. Avast has chosen a different way to differentiate the "home" vs "pro" features. The detection rate in the free is the same as in the paid.
One way to better enable unattended scanning is to download and place in the C: \Windows\System folder a
fake virus (direct download from eicar.org.) You will need to pause the webshield to download it, and probably pause the standard shield if you need to move it. Re-enable them afterward.
This file is a harmless simulation, recognised by any virus scanner, and designed simply to test that the scanner is functioning as it should.
Once it is placed in the folder mentioned - which is one of the first areas to be scanned - shortly after scanning commences, a warning will pop up when the file is encountered. Tick the "do not show this message next time" box, and click "continue". The remainder of the scan will complete without waiting for user action, even if something is found. The scan report can then be viewed later.
Quite a bit on this, and other workarounds, at the Avast forum (it's an often asked question),
here, here, and
here. (And many other places. Includes instructions for getting the task scheduler to use the ashquick.exe module to do it.)