C# ASP.NET MVC'de "InvalidOperationException" hatası

public IActionResult Create(PostDto p,List<string> categories)
        {
            p.UserId = 1;
            p.User = _userService.GetById(1);

            var cs = new List<CategoryDto>();

            foreach (var category in categories)
            {
                cs.Add(_categoryService.GetById(Convert.ToInt32(category)));
            }
            p.Categories = cs;

            ModelState.Remove("User");
            if (ModelState.IsValid)
            {
                _postService.Insert(p);  

            return RedirectToAction(nameof(Index));
            }
            ViewBag.categories = _categoryService.GetAll();
            return View(p);
        }

public void Insert(PostDto post)
        {
            _db.Posts.Add(post.DtoToPost());

            _db.SaveChanges();
        }

public static Post DtoToPost(this PostDto p)
        {
            return new Post { Id = p.Id, Categories = p.Categories.DtoListToCategoryList(),
 Content = p.Content, CreatedAt = p.CreatedAt, DeletedAt = p.DeletedAt, Title = p.Title,
 UpdatedAt = p.UpdatedAt, UserId = p.UserId, User = p.User.DtoToUser() };
        }
public static List<Category> DtoListToCategoryList(this List<CategoryDto> p)
        {
            List<Category> dtos = new List<Category>();
            foreach (var item in p)
            {
                dtos.Add(item.DtoToCategory());
            }
            return dtos;
        }


public static Category DtoToCategory(this CategoryDto c)
        {
            return new Category { Description = c.Description, Name = c.Name, Id = c.Id, Slug = c.Slug };
        }

public static User DtoToUser(this UserDto u)
        {
            return new User { City = u.City, Email = u.Email, Id = u.Id, Name = u.Name, Password = u.Password, Phone = u.Phone };

        }