Fluent API equivalent for Data Annotation

Question

I am not able to use DataAnnotations for setting key and foreign key as I am working on .NET framework. I need to confirm the fluentAPI equivalent of the same.

Below is what i tried, but says "The ALTER TABLE statement conflicted with the FOREIGN KEY SAME TABLE constraint"

model:

public class Item
{
   
    public string ItemNo{ get; set; }
    public string parentItemNo{ get; set; }

  // Adding below two navigation properties to existing model
   public virtual Item Parent { get; set; }
   public virtual ICollection<Item> Children { get; set; } = new List<Item>();
 
}

DBContext:

        modelBuilder.Entity<Item>()
            .HasKey(e => e.ItemNo )
              .HasName("Item$0");
        
         // Adding below lines to create foreign key using fluent API
         entity.HasOne(e => e.Parent)
            .WithMany(e => e.Children)
            .HasForeignKey(e => e.ParentItemNo );