What does Package <package> has no installation candidate mean?

Question

What does the above Error mean? What is an installation candidate?

I was trying to do

sudo apt-get install munin-memcached  

and I get this message:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package munin-memcached is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package munin-memcached has no installation candidate

I googled for the problem and someone said to do a apt-get upgrade but it still did not solve my problem.

Answer 1

This happens when you are trying to install a package about which APT does not have any idea. When you add software sources and then do an apt-get update your system APT's database is updated with all the packages on the repositories listed in software sources list.

Then when you try to install any package, apt checks the package name in it's database, finds it and checks the name of the repo from where it got it. It then downloads the package from that repo.

This means of all the repositories listed in your software sources, none of them have the package named munin-memcached. I checked again and it is not available in official ubuntu repos.

After a bit of googling I found that it is available in a PPA

https://launchpad.net/~chris-lea/+archive/munin-plugins

Add this PPA and install the packages at your own risk. I do not know this person.

To Add the PPA issue below command:

sudo add-apt-repository ppa:chris-lea/munin-plugins

To install package munin-memcached issue below command:

sudo apt-get update && sudo apt-get install munin-memcached

Answer 2

That means that some installed package is saying that it needs a package called munin-memcached to work. But the package manager doesn't find it in the repositories. It is usually followed by some info about what packages are missing it, but it didn't here.

There is no package name munin-memcached in the standard Ubuntu repos, so you are likely using some extra repository. If you post it here we can try to found the cause.