This error message usually comes about after an installation is incomplete for
some reason, meaning that the package manager is stuck and can't proceed until
the damage is fixed. You need to open the terminal and type sudo before the
command. This requires that you enter your password since it is making a change
to the system.

For future reference type sudo before any command that requires root or admin
privileges. This is peculiar to Ubuntu and its derivatives. In other distros you
need to change user to root by typing su plus enter then inputting your
password. then you can type any commands and all will be as root. I only
mention the two ways because you may encounter advice for each and I want you to
know that Ubuntu uses a different scheme to accomplish the same thing, all in
the name of security.