Validate DropDownList in ASP.Net Core MVC

when I run the code, I dont see the error message that Job Title is required.

However, I see the error message saying "First name is required". I also tried to put the bleow code in the controller:

I have the following code:

Model:

public partial class EmployeeInfo
{
    [DisplayName("First Name")]
    [Required(ErrorMessage = "First Name is required")]
    public string FirstName { get; set; } 
    [Required(ErrorMessage = "Job Title is required.")]
    [DisplayName("Job Title")]
    public virtual JobTitleLookup JobTitleLookup { get; set; }
}

public class JobTitleLookup
{     
     public int JobTitleLookupId { get; set; }     
     public string Title { get; set; } = null!;
     public virtual List<EmployeeInfo> EmployeeInfos { get; } = new List<EmployeeInfo>();
}

This is the controller:

public async Task<IActionResult> Index()
{
     jobTitle = await _Reassign.GetJobTitle();
     ViewData["Jobs"] = new SelectList(jobTitle, "JobTitleLookupId", "Title");
}

this is inside the service

public async Task<List<JobTitleLookup>> GetJobTitle()
{
     return await _ackContext.JobTitleLookup.OrderBy(c => c.Title).ToListAsync();
}

this is the view:

 <div class="col-md-4">
        <label asp-for="JobTitleLookup" class="control-label" style="font-weight:bold;"></label>
        <select asp-for="JobTitleLookup" class="form-control" asp-items="ViewBag.Jobs">

            <option value="0">--Please select--</option>
        </select>       
        <span asp-validation-for="JobTitleLookup" class="text-danger"></span>
</div>

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Index(EmployeeInfo info)
{
        if (info.JobTitleLookupId ==null)
        {
            var validationMessage = "Job Title is required";
            this.ModelState.AddModelError("JobTitleLookupId", validationMessage);
        }
        if (ModelState.IsValid)
        {


        }
        return RedirectToAction(nameof(Index));
}

Still, I dont see the error message. Any help with this will be highly appreciated.