Recompiling problem

One of my friends was trying MCreator for the first time. He created only a sword and its recipe, but, when he tried to start Minecraft to test it, an error message showed up saying "Your workspace contains mod that don't compile properly.

So, to try to fix it I exported his workspace and imported to my MCreator, but, I don't have any problem like him. He tried to recreate the workspace and re-import his elements, or reinstalling MCreator, but the error persist.

Any idea what cause it and how to solve it?