The first of the two errors has nothing to do with the package itself - packages need root permisions to be managed, which is what exactly the error is telling you:
E: Unable to lock the administration directory (/var/lib/dpkg/), are you root?
^^^^^^^^^^^^^
I'm aware that the first errors don't have anything to do with the package itself, but this was the command line given. I was wondering what kind of setup might lead to an apt-get command (without sudo even) to display that error message.
Regarding the second, which is instead the core of the problem, see the man page (http://manpages.ubuntu.com/manpages/xenial/man1/purge-old-ke...):
Therefore the correct (and full) command is: As pointed out in another comment, such package is arguably a poor placement for this type of utility.