avast comes with AOS (avast Online Security) which comes as a plug-in/add-on for browsers.
It is perfectly normal that when you install avast a browser that checks things is asking if you want to allow the plug-in/add-on to be installed.
Just allow it.
If you don't want the AOS part, you can remove it as you can with any component in avast.
As this is you first avast install, make sure everything from another (pre)installed av is removed completely.