Asp.net – how the Visual studio not accepting my code

using System;
using System.ComponentModel.DataAnnotations;
using Microsoft.AspNetCore.Mvc;
using Simple_website.Services

namespace simple_website.Controllers
{
    public class UserViewModel
    {
        public Guid Id { get; set; }

      
        public string Login { get; set; }

        public string FirstName { get; set; }

        public string LastName { get; set; }
    
    public string Tittle { get; set; }

        public string ErrorMessage { get; set; }

        public string CreationMode { get; set; }
    }

    public class UserController

    {
        private IUserService _userService;

        public UserController(IUserService userService)
        {
            _service = userService;
        }

        [HttpGet]
        public IActionResult Create()
        {
            ViewBag.Title = "Create a new user";
            return View(new UserViewModel());
        }

        [HttpGet]
        public IActionResult Edit(Guid userId)
        {
            ViewBag.Title = "Edit a user";

            var user = _userService.GetUser(userId);
            if (user == null)
            {
                ViewBag.ErrorMessage = $"User '{userId}' was not found!";
                return View("Error");
            }

            var viewModel = new UserViewModel
            {
                Id = user.Id,
                Login = user.Login,
                FirstName = user.FirstName,
                LastName = user.LastName
            };

            return View(viewModel);
        }

        [HttpPost]
        public IActionResult Save(UserViewModel viewModel)
        {
            if (string.IsNullOrEmpty(viewModel.Login))
            {
                ViewBag.ErrorMessage = "Login is required!";
                return View(viewModel);
            }

            if (ModelState.IsValid)
            {
                if (viewModel.Id == Guid.Empty) // Creation mode
                {
                    var newUser = new User
                    {
                        Login = viewModel.Login,
                        FirstName = viewModel.FirstName,
                        LastName = viewModel.LastName
                    };
                    _userService.CreateUser(newUser);
                }
                else // Edit mode
                {
                    var user = _userService.GetUser(viewModel.Id);
                    if (user == null)
                    {
                        ViewBag.ErrorMessage = $"User '{viewModel.Id}' was not found!";
                        return View("Error");
                    }

                    user.Login = viewModel.Login;
                    user.FirstName = viewModel.FirstName;
                    user.LastName = viewModel.LastName;

                    _userService.UpdateUser(user);
                }

                return RedirectToAction("Confirmation");
            }

            ViewBag.Title = viewModel.Id == Guid.Empty ? "Create a new user" : "Edit a user";
            return View(viewModel);
        }

        [HttpGet]
        public IActionResult Confirmation()
        {
            return View();
        }
    }
}

having error of:

the name ViewBag does not exist in the current context.

the name View does not exist in the current context.

the name _userService does not exist in the current context.

userviewModel does not contain definition for firstName

userviewModel does not contain definition for lastName

the name Modelstate does not exist in the current context.

UseViewModel does not contain a definition for FirsrName and no accessible extension ,method FistName accepting a first argument of type UserviewModel could not be found(are you missing a using directive or an assembly reference?)