I have only recently installed ubuntu 18.04 on one of my machines as a means of becoming competent in using a Linux system, when I attempt to use the command
sudo apt-get update
I receive an error message saying
Could not resolve 'security.ubuntu.com'
and likewise for all four of the main repositories. If it is necessary for me to print the exact output just let me know..
I was able to update upon reboot of my system, however only partially, receiving the following error message in my output:
E: The repository 'http://security.ubuntu.com bionic-security Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.