The backing track was pretty sweet and the rap was nice too.
The vocals overall had everything right except your pitching which is pretty much the most important thing.
This could be a great submission if you just practiced your pitching more.
Try putting the vocal melody you have into a sequencer for something like piano and play it slowly singing along and making sure each note you sing blends perfectly with the sequencer and then try it at full speed making sure you're still hitting the notes right.
I struggle with the same problem but you'll be amazed at what a little practice can do for you.