When getting the error like this:
Ign:1 http://tn.archive.ubuntu.com/ubuntu disco InRelease
Hit:2 http://ppa.launchpad.net/openjdk-r/ppa/ubuntu disco InRelease
Ign:3 http://security.ubuntu.com/ubuntu disco-security InRelease
Ign:4 http://tn.archive.ubuntu.com/ubuntu disco-updates InRelease
Err:5 http://security.ubuntu.com/ubuntu disco-security Release
404 Not Found [IP: 91.189.88.152 80]
Hit:6 https://dl.yarnpkg.com/debian stable InRelease
Ign:7 http://tn.archive.ubuntu.com/ubuntu disco-backports InRelease
Err:8 http://tn.archive.ubuntu.com/ubuntu disco Release
404 Not Found [IP: 91.189.88.152 80]
Hit:9 https://artifacts.elastic.co/packages/7.x/apt stable InRelease
Err:10 http://tn.archive.ubuntu.com/ubuntu disco-updates Release
404 Not Found [IP: 91.189.88.152 80]
Hit:11 https://dl.winehq.org/wine-builds/ubuntu bionic InRelease
Hit:12 https://deb.nodesource.com/node_12.x disco InRelease
Err:13 http://tn.archive.ubuntu.com/ubuntu disco-backports Release
404 Not Found [IP: 91.189.88.152 80]
Hit:14 https://repos.influxdata.com/ubuntu disco InRelease
Reading package lists… Done
E: The repository ‘http://security.ubuntu.com/ubuntu disco-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.
E: The repository ‘http://tn.archive.ubuntu.com/ubuntu disco 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://tn.archive.ubuntu.com/ubuntu disco-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://tn.archive.ubuntu.com/ubuntu disco-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.
The repositories for older releases that are not supported (like 11.04, 11.10 and 13.04) get moved to an archive server. There are repositories available at http://old-releases.ubuntu.com.
The reason for this is that it is now out of support and no longer receiving updates and security patches.
I would urge you to consider a supported distribution. If your computer is too old in terms of memory or processor then you should consider a distribution such as Lubuntu or Xubuntu.
If you want to continue using an outdated release then edit /etc/apt/sources.list and change archive.ubuntu.com and security.ubuntu.com to old-releases.ubuntu.com.
You can do this with sed:
sudo sed -i -re 's/([a-z]{2}\.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
then update with:
sudo apt-get update && sudo apt-get dist-upgrade
Sometimes, it might be faster to create backups of your system and reinstall using supported release instead.