Malware infection

If Avast can detect this malware, why does it allow it on my computer in the first place. Why not just block it? See attachment.

-Maybe it arraived on your computer before avast had signature for it?
-Maybe it is a falsepositive?

Anyway “Serhox” is Adware

If you want help, see sticky post at top in Viruses and Worms section

You can report a suspicious/malicious sample (File/Website) here: https://www.avast.com/report-malicious-file.php

Is PUP detection in Avast enabled…!?

It does not belong on my computer, or any other computer, so it's not a false positive.
File belonging or not to a computer has nothing to do with it being a False Positive or not

Serox adware is a java script usually embedded in HTML pages.