How to Destroy a 2D sprite

(Code works in editor, though once the game is built, it stops working)

I am making a 2D side scrolling game, and I have run into a problem. I have code that will instantiate a bullet, and once that bullet collides with anything, it creates a prefab that represents a spark or bullet hit. The problem is, I have code that destroys that "spark" after about a second, and it does not destroy itself.