Where have you been looking ?
It is a component of Avast and has to be installed - AvastUI > Menu > Settings - General > Troubleshooting > Add/Modify Components.
Then it would have to be enabled in your browser.
All that looks OK, but this online security extension is not showing on my Avast Secure Browser
like it does on the other browsers. A Security Centre & Privacy is, but that is not the same.
All done, extension now added, but it came via a CHROME link. As I already have the extension on my Chrome,
I had ignored adding it to Chrome, but it added it to my Avast browser.
All OK now,