“Unable to locate package” while trying to install packages with APT

There are many questions about this topic. Here I provide a basic/general answer.

When apt-get install is unable to locate a package, the package you want to install couldn’t be found within repositories that you have added (those in in /etc/apt/sources.list and under /etc/apt/sources.list.d/).

The following (general) procedure helps to solve this:

1. Make sure you have enabled Ubuntu repositories: To enable all repositories (main, universe, restricted, multiverse), use the following commands:
sudo add-apt-repository main
sudo add-apt-repository universe
sudo add-apt-repository restricted
sudo add-apt-repository multiverse
   Visit Help for more information.
   
   
2. For finding PPA for more packages:
   • Go to Ubuntu Package Search. ^{(Already explained in this answer)}
   • For External Repositories, Visit Ubuntu Updates and search by button. or Visit PPAs.
   • Or Search in Launchpad ppa
   • Find appropriate ppa according to your Ubuntu release version.
     
3. Add PPA (by command-line): Use this command:
sudo add-apt-repository ppa:<repository-name>
   Visit Ubuntu community help for more information.
   
   
4. Don’t forget to update (make apt aware of your changes): It is essential to run this command after changing any repositories: sudo apt-get update Selecting best download server may help to speed up update.
   
   
5. Finally install the package:
sudo apt-get install <package>
   Refer to Package management by commandline. Additional/Tip: you can find the correct package-name (i.e the name in the repository) using
apt-cache search <package-name>.

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• Related frequently asked Q&A:
  1. How do I resolve unmet dependencies after adding a PPA?
  2. What does Package <package> has no installation candidate mean?

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Note: If package is not available on repository any how, than you have to wait until it is available (in the case of new/updated versions) or use other installation processes than apt-get e.g. compiling from source, downloading executable binary, etc.