Lines and planes in the mathematical sense are 1 and 2 dimensional. They don't have any height (and lines also no width). So they can't exist as a physical object made out of atoms as they are already 3 dimensional.
The fact that something isn't a 3d object doesn't mean it doesn't exist. Does a line of contrast between 2 colors exist? Does a movie projected at a wall exist?
I'd say no. And even if it did, those colours are made out a material that consists of atoms that reflect light, both of which are "fuzzy" and 3D and can't make a proper line.
Does a movie projected at a wall exist?
Sure. There is photons bouncing of a wall and the information they carry we call "the movie". I guess that counts. But the relevant bit is the wall and again it's made out of atoms and therefore is not a proper flat 2D surface.
So yeah, I'd say not being 3D does mean something can't exist in the physical world.