Add Entity without nested entities

I'm working with EF 6 and I need to create a model ignoring nested properties.

I have a model like this (not the same but it's similar)

public class Client
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int? IdEnt { get; set; }
    public Enterprise Enterprise { get; set; }
    public int? IdContact { get; set; }
    public Contact DirectContact { get; set; }
    public Client(string Name, Enterprise enterprise, Contact contact)
    {
        this.Name = Name;
        Enterprise = enterprise;
        DirectContact = contact;
    }
}

public class Contact
{
    public int IdContact { get; set; }
    public string Phone { get; set; }
    public string Email { get; set; }
}

public class Enterprise
{
    public int IdEnt { get; set; }
    public string NameEnt { get; set; }
    public string Dir { get; set; }
}

Contacts are created when I create a new Client, but Enterprise is an entity that exist in DB, so I only need to connect this properties but when I add a new Client and save changes appears an error indicate I try to insert an Enterprise (DB has automatic primary key in Enterprise table)

This is the transaction that I use to create a Client

            bool transaction = false;
            string msj = "";
            var strategy = Context.Database.CreateExecutionStrategy();
            strategy.Execute(() =>
            {
                using var dbContextTransaction = Context.Database.BeginTransaction();
                try
                {
                    Context.Set<Client>().Add(client); // <= client is created with constructor before this part of code
                    Context.SaveChanges();
                    dbContextTransaction.Commit();
                    Transaccion = true;
                }
                catch (Exception e)
                {
                    dbContextTransaction.Rollback();
                    msj = e.InnerException.ToString();
/*
Exception:
Cannot insert explicit value for identity column in table 'X' when IDENTITY_INSERT is set to OFF.

I cannot set off this cause I only need connect the entities.
*/
                    msj = e.Message;
                }
            });

How I can this?

I'm sorry for my explication and my english if i said something wrong.

Extra: I configured the entities with this code:

    public class ClientConfig : IEntityTypeConfiguration<Client>
    {
        public void Configure(EntityTypeBuilder<Cliente> builder)
        {
            builder.ToTable("U_Clients");
            builder.HasKey(ent => ent.ID);
            builder.Property(i => i.ID)
                .HasColumnName("IdClient");
            builder.HasOne(ent => ent.Contact)
               .WithMany()
                 .HasForeignKey(e=>e.IdContact);
            builder.HasOne(ent => ent.Enterprise)
               .WithMany()
                 .HasForeignKey(e=>e.IdEnterprise);
        }
    }