I got this error in Lyx working on a book project:
The layout file requested by this document,
is not usable. This is probably because a LaTeX
class or style file required by it is not
available. See the Customization documentation
for more information.
LyX will not be able to produce output.
If you get this, you’re missing a single package. Fix it with:
sudo apt-get install tetex-extra
The above solution only works for Ubuntu 10.04 and older. For 10.10 (Maverick) do this:
sudo apt-get install texlive-lang-polish
I also had to go to Tools -> Reconfigure after the package installed.