LÖVE

Yeah, that's what the error traceback is good for. It will give you the chain of function calls that led to the error, and often in errors like this it's simply the previous step. Like the error you posted signals that line 21 in your state/level.lua might be the source, because it's the last place before the error on line 9 in entities/score.lua occurs. Perhaps I could've specified that more clearly, but that's basically what I was hinting at.