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

This happens when you try to install a package about which APT does not have any idea. When you add software sources, you need to do an apt-get update so that your system APT's database is updated with all the packages on the repositories listed in the software sources lists.

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 this PPA: chris-lea/munin-plugins

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

To add the PPA, issue the below command:

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

To install package munin-memcached, issue the below command:

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

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.