cant create world because it thinks i dont have the elements to play it?!

For some reason I can no longer create worlds because mcreator said's "I don't have the elements to play it", but I haven't deleted anything and all the elements that it says are missing are still there. This really sucks because now I need to download it and play it on the real Minecraft launcher to see any bugs that are in the mod im making. PLEASE HELP!