When key pressed "missing dependencies"??

I'm using the 1.12 version as I'm making a mod for a modpack. I'm making a gun mod, so when you press R you load a single bullet into the gun. To do this, it checks for a bullet and the 'version' of the gun with a set amount of ammo, and if you have a bullet it will play a sound and swap the gun for one with 1 more bullet in. However, once I make this script in the code blocks, it says "missing dependencies" and is just grayed out and won't even let me use it. Why is this happening? Is there any way to override it