Show a DropdownList in an MVC C # view using Entity Framwork-

I am trying to show a dropdownlist, of an entity that I have in my model, for that I used entityframework database first.

Code in my view:

<div class="col-md-10">
                @Html.DropDownListFor(x => x.Nivel.NivelID, new SelectList(Model.ListNivel,"NivelID","Nivel1"), htmlAttributes : new { @class = "form-control" })                   
                @Html.ValidationMessageFor(x => x.Nivel.NivelID, "", new { @class = "text-danger" })

            </div>

My model:

 public partial class Nivel
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Nivel()
    {
        this.Detalle_Curso = new HashSet<Detalle_Curso>();
        this.Seccion = new HashSet<Seccion>();
    }

    public int NivelID { get; set; }
    public string Nivel1 { get; set; }
    public string Descripcion { get; set; }

    //public List<Nivel> SelectListItems { get; set; }


    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Detalle_Curso> Detalle_Curso { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Seccion> Seccion { get; set; }
}

As I am trying to reference several models in the same view, create a class to be able to reference them.

 public class CursoxGrado
{
    public Nivel Nivel {get; set;}
     public List<Nivel> ListNivel { get; set; }

    public Grado Grado {get; set;}

    public Año_Academico Año_Academico { get; set; }

    public Curso Curso { get; set; }
}

I just create a List of my entity, but when I execute it sends me the following error.