Help states it should be at Scan → Scan for viruses → Quarantine (Virus Chest)
but there is no such option under Scan for viruses
Open Avast
Right click the small lines on the right as shown
From the drop down list select virus chest
It is now on the front page 
You can also check Avast faq: Using the Virus Chest
https://www.avast.com/en-us/faq.php?article=AVKB21#artTitle
It is there when you click the Scan for viruses, that window allows you to select what scan to run, but at the bottom of that window is the ‘Quarantine (Virus Chest)’ text link, hover over it and it turns to a blue highlighted area.
As essexboy suggests if you use the Virus Chest frequently it is easiest to make a shortcut.