The Issue
Running sudo apt-get update or sudo apt update fails with the following or similar errors:
Hit:1 http://apt.postgresql.org/pub/repos/apt eoan-pgdg InRelease Ign:2 http://archive.ubuntu.com/ubuntu eoan InRelease Ign:3 http://archive.ubuntu.com/ubuntu eoan-updates InRelease Ign:4 http://archive.ubuntu.com/ubuntu eoan-backports InRelease Ign:5 http://archive.ubuntu.com/ubuntu eoan-security InRelease Err:6 http://archive.ubuntu.com/ubuntu eoan Release 404 Not Found [IP: 91.189.88.142 80] Err:7 http://archive.ubuntu.com/ubuntu eoan-updates Release 404 Not Found [IP: 91.189.88.142 80] Err:8 http://archive.ubuntu.com/ubuntu eoan-backports Release 404 Not Found [IP: 91.189.88.142 80] Err:9 http://archive.ubuntu.com/ubuntu eoan-security Release 404 Not Found [IP: 91.189.88.142 80] Reading package lists... Done E: The repository 'http://archive.ubuntu.com/ubuntu eoan Release' no longer has 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. E: The repository 'http://archive.ubuntu.com/ubuntu eoan-updates Release' no longer has 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. E: The repository 'http://archive.ubuntu.com/ubuntu eoan-backports Release' no longer has 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. E: The repository 'http://archive.ubuntu.com/ubuntu eoan-security Release' no longer has 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.
The Cause
Older releases of Ubuntu are moved to an archive server, so you need to update your repo lists.
The Fix
# backup your sources file
cp /etc/apt/sources.list /etc/apt/sources.list.bak
# replace the links with the archive address
sudo sed -i -re 's/([a-z]{2}.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
# run update again
sudo apt-get update && sudo apt-get dist-upgrade
Regards