I am assuming (you wouldn't tell us) that you tried to write configuration files for the KSP system and that you screwed up somehow. The error message is actually telling you in clear terms that the gravity model for Eeloo is missing or corrupted.

You've got to carefully Read The Friendly Manual and fix your configuration.

This is not a bug in Principia, this is a bug in your configuration.

Hi, fresh install on Ubuntu, no Kopernicus or anything that modifies planets, yet this error still occurs.

Then we'll need your INFO log as explained in the FAQs.

Also, do you have other mods installed?