is Avast ok on my mac

I’m on the other side of the fence. I use Avast quite regularly and have had no issues with it so far. Even though viruses and other malware are rare on Macs, it’s a good idea to have some sort of antivirus software, provided that the antivirus vendor is reputable.

Something that doesn't exist can't be blocked. A virus is self-propagating software. It requires no user interaction to spread. These have yet to exist in any form in OS X/macOS. Every type of potential malware out there are Trojans. Something the user has to knowingly, or unknowingly install. It can't get on your Mac on its own.

Avast, and any other such software is a complete waste of time and system resources. They can't block Trojans (of which all adware falls under the same type of installation category). The AV software can't know something is dangerous until you install it. And it's pretty useless to tell you the software is bad after the fact.

I have seen multiple topics on these forums where the user has two, or even three types of AV software on their barely-functioning Macs, and they're loaded with adware and other junk. That should give you an idea how pointless AV software is.

macOS has multiple layers of protection built-in to it. It needs no help protecting itself. If anything happens to get by Apple's defenses, it will get by third party junk, too.

I got here a bit late and Kurt has covered your question well, but I will answer your question with an Absolutely Not.

Viruses are not rare, they are non-existent for a Mac.

And Avast is very bad with things that are just malware, the worst of both worlds really.