IMO this thread is getting a bit too micro, you are kind of posting an error after error and I think you should step back and just post your entire module source code, and state the end goal.
It also sounds like you haven’t practiced Elixir enough if lack of mutability and the lexical scope are still surprising for you. Exercism requires some understanding of the language’s constructs. Without that you’ll just be crashing into one error after another, as it seems it is happening currently.
