does not let me create in the entitty framework

I have a problem creating a new headquarters = building.

Pressing the save button in the create view gives me the following error:

  

Entities in 'Entities.RRHH_SEDE' participate in the   'FK_RRHH_MUNICIPIO' relationship. 0 related 'RRHH_MUNICIPIO' were   found 1 'RRHH_MUNICIPIO' is expected

My driver is as follows:

public ActionResult Create()
{
    return View();
}

// POST: RRHH_SEDE/Create
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(mod.RRHH_SEDEModel collection)
{
    try
    {
        // TODO: Add update logic here
        ent.RRHH_SEDE entidad = Mapper.Map<mod.RRHH_SEDEModel, ent.RRHH_SEDE>(collection);
        db.Modificar(entidad);
        return RedirectToAction("Index");
    }
    catch
    {
        return View();
    }
}

I have added the municipality fields in the model but it does not let me save.

public class RRHH_SEDEModel 
{ 
    public short COD_SEDE { get; set; }
    public string NOMBRE { get; set; } 
    public string UBICACION { get; set; } 
    public virtual RRHH_MUNICIPIO RRHH_MUNICIPIO { get; set; } 
    public virtual SEG_ESTADO SEG_ESTADO { get; set; }    
 } 

that's my model