What does HTTP Error 403.14 mean?

Well, I have a Web Api, which configured as follows:

WebApiConfig.Cs

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // Web API routes
        config.MapHttpAttributeRoutes();

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

It's my GlobalAsax

public class WebApiApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        GlobalConfiguration.Configure(WebApiConfig.Register);
    }
}

However, for some reason it is not finding the files. And generates the following problem:

HTTP Error 403.14 - Forbidden The Web server is configured not to list the contents of this directory.

Most likely causes:

A default document is not configured for the requested URL, and the search in > directory is not Enabled on the server.

Why does this happen?

How can I solve this problem?