For future reference @NobbZ said
the :erlang.apply/3 is called with wrong kind of arguments. Usually the first argument should be a module name.
So usually the problem is that you might be invoking the Guardian.encode_and_sign_in function without alliasing MyApp.Guardian and that’s why you get this wierd looking error so either alias MyApp.Guardian at the start of your file or just use MyApp.Guardian.encode_and_sign_in.
