Validation error asp core – The field is required

have a project that I am studying, it consists of a small mvc web store with asp core.
I followed the course, however, the version used in the videos is 2.1 and I have 5,6,7.
The error consists of ModelState.IsValid.
The problem is that my create.cshtml form is requiring a field that I don’t have in the form. I have a DepartmentId field but it asks for Department. My classes and the error.
using System.ComponentModel.DataAnnotations;

Seller.cs

namespace LojaWebMvc.Models;

public class Seller
{

    public int Id { get; set; }
    [Required(ErrorMessage ="{0} required")]
     [StringLength(60, MinimumLength = 3, ErrorMessage ="{0} size shold be between {2} and {1}")]
    public string Name { get; set; }
    [Required(ErrorMessage ="{0} required")]
    [EmailAddress(ErrorMessage = "Enter a valid email")]
    public string Email { get; set; }
    [Display(Name="Birth Date")]
   [DataType(DataType.Date)]
   [Required(ErrorMessage ="{0} required")]
    public DateTime BirthDate { get; set; }
    [Display(Name = "Base Salary")]
   [DisplayFormat(DataFormatString ="{0:F2}")]
    [Required(ErrorMessage ="{0} required")]
    [Range(100.0, 5000.0, ErrorMessage ="{0} must be from {1} to {2}")]
     public double BaseSalary { get; set; }
    public Department Department {get; set;}
    public int DepartmentId { get; set; }

    public ICollection<SalesRecord> Sales {get; set;} = new List<SalesRecord>();

    public Seller()
    {

    }
    public Seller(int id, string name, string email, DateTime birthdate, double salary, Department department)
    {
        Id = id;
        Name = name;
        Email = email;
        BirthDate = birthdate;
        BaseSalary = salary;
        Department = department;

    }
    public void AddSales(SalesRecord sr)
        {
            Sales.Add(sr);
        }

        public void RemoveSales(SalesRecord sr)
        {
            Sales.Remove(sr);
        }

        public double TotalSales(DateTime initial, DateTime final)
        {
            return Sales.Where(sr => sr.Date >= initial && sr.Date <= final).Sum(sr => sr.Amount);
        }
}

Department.cs

namespace LojaWebMvc.Models;

public class Department
{
       public int Id { get; set; }
        public string Name { get; set; }
        public ICollection<Seller> Sellers { get; set; } = new List<Seller>();

        public Department()
        {
        }

        public Department(int id, string name)
        {
            Id = id;
            Name = name;
        }

        public void AddSeller(Seller seller)
        {
            Sellers.Add(seller);
        }

        public double TotalSales(DateTime initial, DateTime final)
        {
            return Sellers.Sum(seller => seller.TotalSales(initial, final));
        }
}

DepartmentService FindAll:

...
public List<Department> FindAll()
 {
    //return _vsproContext.Department.OrderBy(x => x.Name).ToList();
    var list = _vsproContext.Department;
    var obj = new List<Department>();
    obj = list.Select(x => new Department(
        x.Id,
        x.Name  
    )).ToList();
    return obj;
  }
...

SellerService:

using LojaWebMvc.Models;
using LojaWebMvc.Services.Interfaces;
using LojaWebMvc.Data;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using LojaWebMvc.Services.Exceptions;

namespace LojaWebMvc.Services;

public class SellerService: ISellerService
{
    private readonly VsproContext _vsproContext;
    public SellerService(VsproContext vsproContext)
    {
        _vsproContext = vsproContext;
    }
//
        public List<Seller> FindAll()
        {
           // return _vsproContext.Seller.OrderBy(x => x.Name).ToList();
            var list = _vsproContext.Seller;
            var obj = list.Select(x => new Seller(
                x.Id,
                x.Name,
                x.Email,
                x.BirthDate,
                x.BaseSalary,
                x.Department

            )).ToList();
            return obj;
        }

        public Seller FindById(int id)
        {
            return _vsproContext.Seller.Include(obj => obj.Department).FirstOrDefault(x => x.Id == id);
               // var obj = _vsproContext.Seller.Find(id);
               // return obj;

        }

        public Seller Insert(Seller seller)
        {
            _vsproContext.Add(seller);
            _vsproContext.SaveChanges();
            return seller;
        }

        public void Remove(int id)
        {
            var obj = _vsproContext.Seller.Find(id);
            _vsproContext.Remove(obj);
            _vsproContext.SaveChanges();

        }

        public void Update(Seller seller)
       {
            if(!_vsproContext.Seller.Any(x => x.Id == seller.Id))
            {
                throw new NotFoundException("Error - Id not found");
            }
            try{
            _vsproContext.Update(seller);
            _vsproContext.SaveChanges();
            }catch(DbUpdateConcurrencyException e) 
            {
                throw new DbConcurrencyException(e.Message);
            }

       }


    
}

Controller:

[HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult Create(Seller seller)
    {
      
        if(!ModelState.IsValid)
        {
      
           var message = string.Join(" | ", ModelState.Values
            .SelectMany(v => v.Errors)
            .Select(e => e.ErrorMessage));
            System.Console.WriteLine(ModelState.Values);
            var departments = _departmentService.FindAll();
            var viewModel = new SellerFormViewModel{ Seller = seller, Departments = departments};
            System.Console.WriteLine(message);
            return View("Create",viewModel);
        }
         
        
        _sellerService.Insert(seller);
        return RedirectToAction("Index");

    }

If you remove the if(!ModelState.Isvalid)… It works normally, but if I try to validate, it doesn’t save it in the database and doesn’t return any error. I get stuck. I got the error output using System.Console.WriteLine(message)

Output error: The Department field is required.

Remove if(!ModelState.Isvalid) its work fine.

>Solution :

you can try to add ? manully to like:

public Department? Department {get; set;}

or:
you can remove <Nullable>enable</Nullable> from your project file (double-click the project name or right-click the project to choose Edit Project File). You can read this to know more.