ModelState.AddModelError dosen't show error

[HttpPost]
       public ActionResult Register(tbl_User model)
       {
           if (ModelState.IsValid)
           {
               if (MyClass.IsUserNameDuplicate(model.username) == true)
               {
                   this.ModelState.AddModelError("username", "user name is duplicate");

                   return View(model);
               }
               else
               {
                   MyClass.creatUser(model.username, model.password_User, model.nam);
               }
           }

           return View(model);

       }

@Html.ValidationSummary(true)
@Html.ValidationMessageFor(model => model.username)
@Html.TextBoxFor(m => m.username, new {@class = "input", @placeholder = "*enter your username" })