You do not run as root as in Debian or Fedora in Ubuntu where you can log on
as root or switch user to root. In Ubuntu you use sudo before the command
that requires root permissions. In this case preface your alien command with
the word sudo.

sudo alien -d package-name.rpm

Sorry I just noticed the format you are using is not rpm. Oops!

You cannot convert tar gk pk to deb. You can only convert rpm to deb with
alien. If there is an rpm inside the compressed file then you must extract
it first. If it is just source code then you do NOT use alien, but need to
compile the source code instead. That is a whole different process.

Not strictly true. Although the vast majority of people use alien to
convert from .rpm to .deb - and that's pretty much *all* it's used for
in Ubuntu - it's capable of a lot more.

From the alien manpage:

"alien is a program that converts between Red Hat rpm, Debian deb,
Stampede slp, Slackware tgz, and Solaris pkg file formats. If you want
to use a package from another linux distribution than the one you have
installed on your system, you can use alien to convert it to your
preferred package format and install it. It also supports LSB packages."

The default is to convert the package to .deb format - the parent
package format is automatically detected - but it can convert from/to
any of the listed package formats except LSB, which it can only convert
from, not to.

to run as root type sudo before your command. You could type sudo su
that will set you as root till you type exit but unless you are running
a bunch of commands that need root status and work slowly, you are best
just typing sudo at the start of the line.