I do not recognize the reference of my class library

Hello friends I am working on 3 layers with WebForms and I am already in the presentation layer, I try to run the layer in some browser and I get this error:

  

CS0246: Can not find the type or space name of   'Business' names (missing a using directive or a reference from   assembled?)

     

Line 6: using System.Web.UI;

     

Line 7: using System.Web.UI.WebControls;

     

Line 8: using Business;

     

Line 9:

     

Line 10: public partial class _Default: System.Web.UI.Page

This is the code of my presentation layer

using System;  using System.Data;   using System.Collections.Generic;   using System.Linq;   using System.Web;     using System.Web.UI;    using System.Web.UI.WebControls;    using Business;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

        if (!Page.IsPostBack)
            gvMostrar(sender, e);

    }

    protected void Agregar(object sender, EventArgs e)
    {
        if (!Page.IsValid)
            return;
        AccesoLogica negocio = new AccesoLogica();
        string nombre = txtNombre.Text;
        string apellido = txtApellido.Text;
        int edad = Int32.Parse(txtEdad.Text);
        int resultado = negocio.Insert(nombre, apellido, edad);
        if (resultado > 0)
            lblMensaje.Text = "Nuevo Registro Agregado Satisfactoriamente.";
        else
            lblMensaje.Text = "Nombre: [" + txtNombre.Text + "] ya existe, agrege otro";
        negocio = null;

    }

    protected void gvMostrar(object sender, EventArgs e)
    {
        GridView.DataSource = AccesoLogica.ObtenerEmpleados();
        GridView.DataBind();
        txtEdad.Text = txtNombre.Text = txtApellido.Text = "";
    }
}

This is the code of my library Business, in which I am using the class ACCESS DATA WHERE I BRING THE METHODS THAT I BELIEVE.

using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks; using System.Data; using TresCapas;

namespace Business {     public class AccesoLogica     {         public static DataTable Get Employees ()         {             return Data.ObtainEmployees ();         }

    public int Insert(string Nombre, string Apellido, int Edad)
    {
        AccesoDatos acceso = new AccesoDatos();
        return acceso.Insert(Nombre, Apellido, Edad);
    }
}

}