How to set a property to be "Identity" or auto-incremented column with Entity Framework Core?

In a new WPF project that I am writing using c#, I want to attempt on using Entity Framework Core to interact with my SQL Server database.

Every time I try to add model to my context, I get the following error

Cannot insert explicit value for identity column in table 'Orders' when IDENTITY_INSERT is set to OFF.

I am using a Repository and UnitOfWork which wraps the Entity Framework Core methods to perform the work needed.

But at its simplest, I am executing the following code

var order = new Order();
order.Title = "some";
....
Context.Orders.Add(order);
Context.SaveChanges();

Here is my model

public class Order
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Int64 Id { get; set; }

    public string Status { get; set; }
    public int? CustomerId { get; set; }
    public DateTime? Birthdate { get; set; }
    public int UtcOffset { get; set; }\
    public DateTime CreatedAt { get; set; }
    public DateTime? UpdatedAt { get; set; }
    public int? UpdatedBy { get; set; }

    [ForeignKey(nameof(Creator))]
    public int CreatedBy { get; set; }

    public Order()
    {
        CreatedAt = DateTime.UtcNow;
    }

    public virtual User Creator { get; set; }
    public virtual Customer Customer { get; set; }
}

What could be causing this problem?

Updated

Here is how my table is created

CREATE TABLE [dbo].[Orders](
    [Id] [bigint] IDENTITY(1,1) NOT NULL,
    [Status] [varchar](50) NOT NULL,
    [CustomerId] [int] NULL,
    [Birthdate] [datetime] NULL,
    [CreatedBy] [int] NOT NULL,
    [CreatedAt] [datetime] NOT NULL,
    [UpdatedBy] [int] NULL,
    [UpdatedAt] [datetime] NULL,
    [UtcOffset] [int] NOT NULL,
 CONSTRAINT [PK_Orders] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

Here is the method that creates the Order model

public Order Create(int? customerId, DateTime? birthdate)
{
    var order = new Order();
    order.CustomerId = customerId;
    order.Birthdate = birthdate;
    order.Status = OrderStatus.Sold.ToString();
    order.CreatedBy = Passport.Identity.Id;

    var updatedOrder = Orders.Add(order);
    Orders.Save();

    return updatedOrder;
}

Here is my repository implementation

public class EntityRepository<TEntity, TKeyType> : IRepository<TEntity, TKeyType>
    where TEntity : class
    where TKeyType : struct
{
    protected readonly DbContext Context;
    protected readonly DbSet<TEntity> DbSet;

    public EntityRepository(DbContext context)
    {
        Context = context;
        DbSet = context.Set<TEntity>();
    }

    public TEntity Get(TKeyType id)
    {
        return DbSet.Find(id);
    }

    public IEnumerable<TEntity> GetAll()
    {
        return DbSet.ToList();
    }

    public bool Any(Expression<Func<TEntity, bool>> predicate)
    {
        return DbSet.Any(predicate);
    }

    public IQueryable<TEntity> Find(Expression<Func<TEntity, bool>> predicate)
    {
        return DbSet.Where(predicate);
    }

    public TEntity SingleOrDefault(Expression<Func<TEntity, bool>> predicate)
    {
        return DbSet.SingleOrDefault(predicate);
    }

    public virtual TEntity Add(TEntity entity)
    {
        DbSet.Add(entity);

        return entity;
    }

    public virtual IEnumerable<TEntity> AddRange(IEnumerable<TEntity> entities)
    {
        DbSet.AddRange(entities);

        return entities;
    }

    public void Remove(TEntity entity)
    {
        DbSet.Remove(entity);
    }

    public void RemoveRange(IEnumerable<TEntity> entities)
    {
        DbSet.RemoveRange(entities);
    }

    public void Update(TEntity entity)
    {
        DbSet.Attach(entity);
        var record = Context.Entry(entity);
        record.State = EntityState.Modified;
    }

    public IQueryable<TEntity> Query()
    {
        return DbSet;
    }

    public void Save()
    {
        Context.SaveChanges();
    }
}

public class EntityRepository<TEntity> : EntityRepository<TEntity, int>
   where TEntity : class
{
    public EntityRepository(DbContext context)
        : base(context)
    {
    }
}

Additionally, this question is not a duplicate on Entity Framework error: Cannot insert explicit value for identity column in table because I am decorating my Id property with the [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]. Moreover, I am using database first approach, so I created my database manually using MSSMS