Hi
im new user of avast android and i am facing this issue also, but i think i know the reason behind this issue
i believe the reason is in the android it self ... the android which have very strict SElinux which prevents regular user nor outside system apps to make changes on the system partition in the android ... now i don't know about others devices but my device is HTC One with selinux enforced ... the only solution is to install by custom recovery
but what does avast want to install and where ?