It's fixed! I saved it as an STL file (with the STL extension) and it worked. I guess some file types simply aren't compatible.

So I've made the mold, but it doesn't export to Cura very well at all. I used a solid checker extension and all of the pieces are indeed solid.

I get a large yellow block in Cura. I used the "solid tools - subtract" on Sketchup to make the mold from the pieces.

Let's say I have a design for a chess piece, but I want to print out a mold using a 3d printer. I can't do this by simply 3d printing the chess piece. I have to 3d print a mold, which is an "inverse" of the chess piece because it would be a cube, with a hole in the shape of the chess piece. I would then pour the casting media into the mold.

Is there a way to delete an object from another object on sketchup?