[Natsheh]

I mean it will be much easier if you highlighted that line anyways

new skinId = chestsList[chest][chance][skinNum];
This error happens when index of an array gets out of bounds I assume the 1st dimension of the array is out of bounds "chest"

You can contact the author Psychical for the fix or you can try debugging by yourself and check what is going out of bounds and fix it depending on the things that suppose to go correctly.