Recently I have installed an older version of Ubuntu on my old machine. Whenever I try to install any software, I get an error saying it couldn't be found:
$ sudo apt-get install vlc
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package vlc
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If you want to continue using an outdated release then edit /etc/apt/sources.list and change archive.ubuntu.com and security.ubuntu.com to old-releases.ubuntu.com.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.
Source: Can I keep using Ubuntu 9.04 if it's outdated?
Once you have performed the above steps to switch to the old-releases mirrors, update the Update Manager and then do do-release-upgrade:
sudo apt-get update
sudo apt-get install ubuntu-release-upgrader-core
sudo do-release-upgrade
See also EOLUpgrades - Community Help Wiki.
