There are several possible reasons but the first one is that only an
administrator, the root user, can install anything. Try "sudo apt-get ..."
(no quotes or dots, just insert the package name etc.). If you have Synaptic
or anything else open you will get the "can't lock" message, so check
(alt-tab) that you are only using one installation app. Open your browser to
make sure you are indeed connected to the internet, as the message about not
finding a repository is often caused by a lapsed internet connection.

That particular error usually means that you already have a package manager
open. When you close the package manager, the lock is removed. This is to
prevent two package managers from running simultaneously. If you are sure that
you do not have another package manager running or are doing two operations at
the same time then you can remove the lock manually. It is just a file in the
path that you listed. You need to do this as root so will need to use sudo
nautilus or gksu nautilus from the terminal.