Hello Lenik,
I will stick to english just in case someone else encounters a similar problem.
1. I would try to open our uninstaller. It is located in /Applications/Avast.app/Contents/Backend/utils/com.avast.uninstall.app and try to uninstall the AV. It might be that just the UI refuses to open for some reason.
2. If the uninstaller is not there (maybe the entire /Applications/Avast.app might not be there), then I would recommend deleting /Library/Application Support/Avast
You should be able to install Avast again after that.
Please let us know whether you managed to resolve the issue and what helped.
Best regards,
Jakub