I was having an issue in installing package on Release:20.04, running server version, codename: focal, I tried with sudo apt-get update but still, it was not working and I then read on StackOverflow and tried the following
Step-1:Remove the file sources.list
sudo rm -fr /etc/apt/sources.list
Step-2:Run the update process. It will create the file again
sudo apt-get update
Now /etc/apt/sources.list file is deleted and I am unable to install any package.
Thanks