cannot find avast on my mac

#1 I do not recommend running either Avast nor AVG as they are unnecessary and do more harm to the performance of your Mac than preventing infection.

Here's an Avast support page on how to uninstall:

https://support.avast.com/en-ww/article/Uninstall-Mac-Security/

Better to avoid getting infected in the first place:

1. Do not be fooled by phishing emails, text messages, ads telling you that you are infected, etc.
2. Do not pirate software (frequently includes bundled malware)
3. Do not install software from untrusted sources, stick to the Apple Mac AppStore if you can and if you cannot make sure you are going to reputable software company website directly. Be wary of shareware from download.com and other sites as they bundle adware. Be careful installing games from sources other than the AppStore. Avoid running software that Apple warns you is not signed or notarized. Never install a Configuration Profile unless it's your employer or school asking you to do it.
4. Do not use an account with administrative privileges, use a standard account with a secondary admin account. Think twice before submitting the admin account login information when prompted.
5. Avoid the seedy underbelly of the Internet.
6. Do not give others remote control access to your Mac unless you are absolutely certain it is someone you know and trust. Lots of scammers claiming to be Apple or Microsoft, etc. cold calling people using social engineering to trick you into paying outrageous amounts of money. They are scammers, don't interact with them.
7. Keep both macOS and all your applications up-to-date
8. Probably a good idea to use an adblocker as there are malicious advertisements that can auto-download and run malicious software. Do not type your admin account & password into any prompts that appear unexpectedly. As in you are not installing software and one just pops up while web surfing.

It is amazing just how useless AV Software is and just how invasive it can be.

Effective defenses against malware and other threats

There are no known Viruses in the wild that self replicate and affect macOS. There are Malware and Adware that does affect macOS and are often times downloaded as part of an Application from Third Party UnTrusted Site and get installed along with the Application. And for that instance - Suggest downloading from a Trusted Developer and Respected ASC Contributor the application Malwarebytes for Mac. It is free or paid for added features. Run the Application and it should remove the malware / adware. Once done, restart computer and test.

You can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For Avast software you'd do the following search(es): 

1 - Name contains avast

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

You really should remove Avast completely. Try re-downloading Avast and running the installer, in the past they had an option in the installer to uninstall it. It's been a while since I encountered Avast. But you could also try re-installing it and then following the support document to uninstall it.

Try this trusted Application called Find Any File (FAF ) and us the search for Avast.

Falling that, Suggest downloading the Application Etrecheck directly from a Trusted Developer and well Respected ASC Contributor. The application is free or paid from added features. Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ). It will take a Snap Shot -  both the hardware and software. The Report will Not Reveal Any Personal Information. Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Regardless of the operating system, most all security software is an ugly hack after thought. Needing to hook into the kernel, bypass normal application restrictions, etc.

Apple has been raising the bar lately.

Don't install AVG, it's not needed.