Error: no HTTP resource was found that matches the request URI

public class testADOController : ApiController
   {
       [HttpGet]
       public testOrg GetOrgTreeStr(int emp_no)
       {
            return testadoBL.GetTestOrg(emp_no);
       }
   }

config.MapHttpAttributeRoutes();

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

public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        name: "Default",
        url: "{controller}/{action}/{id}",
        defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
    );
}

somelocalhost/api/testado/GetOrgTreeStr/100100

{
Message: "No HTTP resource was found that matches the request URI 'http://localhost:53435/api/testado/GetOrgTreeStr/100100'.",
MessageDetail: "No action was found on the controller 'testADO' that matches the request.",
}