It would have been much easier if you had posted the full errormessage including the module for that String.Chars is not implemented and the line number it is complaining about.
So I have to guess for now where the message does come from, and the sole possbility I do see for now is this line: {:ok, response} -> "response: #{response}". response is of type HTTPotion.Response or HTTPotion..AsyncResponse and after a quick look up in the sources I wasn’t able to find a defimpl for both of them and they don’t seem to have Inspect implemented either (there was not a single defimpl in the repository according to github search).
So If that is really the line causing you trouble, You need to get the information you want to have in the string in another way.
