Well you need only post in the Resources/Support forum if you have a question about why the piece was sent back. The reason, by the way, is because it is just a cube, and the gallery is for finished work, not basic studies of geometric shapes etc. You can also post the the WIP forum for feedback on how to improve your work, but there's not much to say about it. The light pixels in the bottom left portion of the outline need to go, otherwise a cube is a cube, and I'd encourage you to attempt more challenging subject matter. This sort of work is fine for practice of course.