Okay im confused.

You suck at matching the colouring scheme?

They had this painted on the wall at my old school.
For some reason when you move the shapes around a gap appears

its not that hard or am i just super retarded and done the wrong thing?

Too easy. If you look at the top picture the top edge made by the red and green triangles isn't straight (look at where they meet then look at the same point on the bottom picture).
Although it is only very slight, if you added up all the part squares on the top picture and put them together you'd end up with 1 complete square, so when they are re-arranged in the bottom picture (where the top line is straight) it forms that single empty square.