can't access viewdata and viewbag from controller to view

Controller

public ActionResult BookCourse(string Id)
        {
            services();
            var item = _courseDetailsService.GetAll();
            ViewBag.Code = Id.ToString();
            ViewBag.Course = item;
            TempData["code"] = Id;
            ViewBag.title = "Booking Course";
            ViewBag.Flag = false;
            return View();
        }
        [HttpPost]

        [RecaptchaControlMvc.CaptchaValidator]
        public ActionResult BookCourse(CourseBooking book, bool captchaValid, string captchaErrorMessage)
        {
            try
            {
                services();
                string code = TempData["code"].ToString();
                    bool valid = true;
                
                    if (!captchaValid)
                    {
                        ViewBag.ErrorMessage = "Invalid Entry *";
                        ModelState.AddModelError("Captcha", "Please Enter verification text correctly");
                        valid = false;
                    }

                    if (book.FirstName == null || book.FirstName == "")
                    {
                        ModelState.AddModelError("FirstName", "Enter First Name");
                        valid = false;
                    }
                    if (book.LastName == null || book.LastName == "")
                    {
                        ModelState.AddModelError("LastName", "Enter Last Name");
                        valid = false;
                    }


                    if (book.EmailAddress == null || book.EmailAddress == "")
                    {
                        ModelState.AddModelError("EmailAddress", "Enter Email Address");
                        valid = false;
                    }
                    else
                    {
                        string emailRegex = @"^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}" +
                                                 @"\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\" +
                                                    @".)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$";
                        Regex re = new Regex(emailRegex);
                        if (!re.IsMatch(book.EmailAddress))
                        {
                            ModelState.AddModelError("Email", "Email Address is not valid");
                            valid = false;
                        }
                    }
                    if (!valid)
                    {
                        ViewBag.Flag = true;                        
                        return BookCourse(code);
                    }
                    book.DateBooked = DateTime.Now;                
                    _courseBookingService.Add(book);
                    SendNotificationMail(book);                   
            }
            catch(Exception ex)
            {
                _log.Log(ex);
            }            
                return RedirectToAction("Thanks");           
        }
View(Book Course)
 @if ((bool)ViewBag.Flag)
        {
             <div id="note">
                <div class="notification_error">
                    @Html.ValidationSummary(false)
                </div>
            </div><!-- End notification -->
        }