Hello,
it seems you have previously already installed Avast, and have not uninstalled it properly. Or you have used some kind of backup/migration software that did migrate only part of the installation.
I would try deleting it's files by opening Terminal app and running the following two commands in it:
sudo rm -rf /Applications/Avast.app
sudo rm -rf /Library/Application\ Support/Avast
afterwards it would be best to restart your Mac; this should be enough cleanup to allow a subsequent installer to run properly.
Kind regards,
Ondrej Kolacek