Error CS0012 The type 'Client' is defined in an assembly that is not referenced

Class manipulation with EF:

ContextEF Contexto = new ContextEF();

public List<Cliente> Listar()
    {
        //var query = from c in Contexto.Cliente
            //          select c;
        return Contexto.Cliente.ToList() ;
    }

Onclick from the query page

protected void btnPesquisar_Click(object sender, EventArgs e)
    {
        ClientService service = new ClientService();
        var lista = service.Listar();

        gvListacEF.DataSource = lista;
        gvListacEF.DataBind();
    }

Error presented:

  

Error CS0012 The type 'Client' is defined in an assembly that is not referenced. You must add a reference to assembly 'DAO, Version = 1.0.0.0, Culture = neutral, PublicKeyToken = null'