Insert value into identity column in Core 2.2

Trying to seed database in MSSQL server. 'Id' column is set to identity. I fail to understand why EF needs data for 'Id:

public class Location
{
    public int? Id { get; set; }

    public string Name { get; set; }

    public IList<Office> Offices { get; set; }

}

... fluent API:

modelBuilder.Entity<Location>()
     .HasKey(k => k.Id);

modelBuilder.Entity<Location>()
     .Property(p => p.Id)
     .UseSqlServerIdentityColumn()
     .ValueGeneratedOnAdd();

modelBuilder.Entity<Location>()
     .HasData(
            new Location() { Name = "Sydney" },
            new Location() { Name = "Melbourne" },
            new Location() { Name = "Brisbane" }
    );

... as far as I understand 'Id' doesn't need to be provided if it's generated by server on insert. Why do I get the messages about not providing Id ...

I think that the error is here

public int? Id { get; set; }

Id should not be nullable.

Update: What I mean is that you should write:

public int Id { get; set; }

The question mark makes your property nullable, but since it is a primary key it cannot be null.

I did a littel example here:

using System.Collections.Generic;

namespace ConsoleApp2.Models
{
    public class Location
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public IList<Office> Offices { get; set; }
    }
}

Fluent Api

      migrationBuilder.CreateTable(
                name: "Locations",
                columns: table => new
                {
                    Id = table.Column<int>(nullable: false)
                        .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
                    Name = table.Column<string>(nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_Locations", x => x.Id);
                });

I can add new location without problems.

using ConsoleApp2.Models;
using System.Collections.Generic;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            MyDbContext _c = new MyDbContext();

            List<Office> list = new List<Office>()
            {
                  new Office()
                {
                    OfficeName = "Reception"
                }
            };


            Location l = new Location()
            {
                Name = "New York",
                Offices = list
            };

            _c.Locations.Add(l);
            _c.SaveChanges();
        }
    }
}

Im using .net core 2.1 with EFcore 2.2.2.

I hope that help.