If “Lux” (https://play.google.com/store/apps/details?id=com.vito.lux&hl=en) is installed, much of the AMS 5 interface is non-functional, for example Scheduled Scan enable/disable and app permissions. Lux has to be disabled to permit these screens to work.
Just add Avast into Lux’s Sleep List.
Excellent. Problem resolved. Thanks.