You must have admin rights at the moment - not just the possibility to do administration. There are 2 ways how to do it:
from Terminal:
sudo rm -fr /Library/Application\ Support/Avast/files/Setup/filedir/*.locked
from Finder:
press apple+lshift+g , enter the path “/Library/Application Support/Avast/files/Setup/filedir” , and delete those files (Finder will ask you for administrative password).