Error   CS0246  The type or namespace name 'HttpListener' could not be found (are you missing a using directive or an assembly reference?)

UPDATE1

private readonly HttpListener _httpListener;

            if (!HttpListener.IsSupported)
        {
            throw new NotSupportedException(
                "The Http Server cannot run on this operating system.");
        }

        _httpListener = new HttpListener();
        _httpListener.Prefixes.Add(prefix);
        _sessionSettings = settings;