fontspec cannot find fonts on Ubuntu

Your TeX Live installation is faulty or incomplete. It misses the Latin Modern fonts in OpenType format which are necessary for fontspec to work, because they are the “fallback fonts”.

If you have the “vanilla” TeX Live, use tlmgr to install/reinstall

collection-fontsrecommended

(or just the lm package). For a TeX Live/Debian, use apt-get to install all “recommended” packages.

In any case, a full install is preferable than adjusting it when things go wrong. Within TeX Live/Debian it's always difficult to understand what Debian package contains what we need.

This worked for me:

sudo apt-get install lmodern

See XeTeX not working on Ubuntu 10.04?