For future reference, most likely the reason why the current Avast doesn’t work on El Captain is because of System Integrity Protection (more on that in the link below), SIP can be disabled and I bet Avast works with SIP disabled.
AVs work at a low level so this affects AVs directly.
http://www.macworld.com/article/2948140/os-x/private-i-el-capitans-system-integrity-protection-will-shift-utilities-functions.html