First, what's happening is that your function requires 3 arguments, but it will be called with either 2 or 3 arguments. As in many languages, calling a function with less or more arguments than its required amount will throw an error before anything inside the function runs.

If it's not clear yet, the tip here is that you need to look up how to make that 3rd argument optional. I'll update the description to provide that tip as well since it may not be clear.

Keep in mind that you can still unlock the solutions if you want it spoiled. Honestly, I find reading other people's solutions to the same problem as a useful way to learn.