greek encodings under lyx

I had the same error in Ubuntu. A previously working document with greek letters gave the two errors below:

! Package fontenc Error: Encoding file `lgrenc.def' not found 
(fontenc) You might have misspelt the name of the encoding.

l.99 \ProcessOptions*                 
Necessary code for this encoding was not loaded.
Thus calling the encoding later on will produce further error messages.

! LaTeX Error: This NFSS system isn't set up properly.

l.261 \begin{document}
For encoding scheme LGR the defaults cmr/m/n do not form a valid font shape

The package greek-fontenc was missing. I installed the package "TexLive: Greek" from the Ubuntu repository and after that the document compiled properly.

"TexLive: Greek" is the debian package texlive-lang-greek. It contains greek-fontenc but also a bunch of other TeX packages. Perhaps you're missing one of them (e.g. cbfonts, "a complete set of Greek fonts"). See its debian page for the list of TeX packages included.

If anyone encounters this problem under Arch Linux, the equivalent package to install is texlive-langgreek.

For anybody who encounters this problem while using tlmgr instead of the distro package managers, you can solve this problem by installing the following packages:

• greek-fontenc
• babel-greek
• cbfonts