DropDownListFor how to use?

@uitan, this exception is masking the real problem that should be just validating your form. You must load the ViewBags of the post action as you did in the Get action. your post should look like this:

// POST: ServiceProvider/Create
[HttpPost]
public ActionResult Create(ServiceProviderViewModel serviceProvider)
{
    if (ModelState.IsValid)
    {
        var serviceProviderDomain = Mapper.Map<ServiceProviderViewModel, ServiceProvider>(serviceProvider);
        _serviceProviderApp.Add(serviceProviderDomain);
        _serviceProviderApp.Save();

        return RedirectToAction("Index");
    }

    //FALTAM ESSAS LINHAS:
    ViewBag.Position = new SelectList(_positionApp.GetAll(), "PositionId", "Description");
    ViewBag.Departament = new SelectList(_departamentApp.GetAll(), "DepartamentId", "Description");

    return View(serviceProvider);
}

Mark as resolved if it works. :)

Code taking into account that PositionId should be a combo:

Na Action Get:

Obs: You should pass the ViewBag.Position

public ActionResult Create()
{
    ViewBag.Position = new SelectList(_serviceProviderApp.SeuMetodoDeObterPositions(), "PositionId", "SeuCampoDescricao");

    return View();
}

Na view:

Note: This Code places the first option blank and then the data coming from the ViewBag.

@Html.DropDownList("PositionId", (IEnumerable<SelectListItem>)ViewBag.Position, String.Empty, new { @class = "form-control" })

No Domain:

 public class ServiceProvider
{
    #region Attributs
    public int ServiceProviderId { get; set; }
    public string Name { get; set; }
    public string MotherName { get; set; }
    public string FatherName { get; set; }        
    public string Email { get; set; }
    public DateTime Birth{ get; set; }
    public DateTime DateRegister { get; set; }
    public DateTime DateModified { get; set; }
    #endregion
    #region Foreingkeys
    public int DepartamentId { get; set; }
    public int PositionId { get; set; }
    #endregion
    #region Properties Navigations
    public virtual Departament Departaments { get; set; }
    public virtual Position Positions { get; set; }
    #endregion
}

public class Position
{
    #region Attributs
    public int PositionId { get; set; }
    public string Description { get; set; }
    public DateTime DateRegister { get; set; }
    public DateTime DateModified { get; set; }
    #endregion
    #region Relationships Other Class
    public virtual IEnumerable<ServiceProvider> ServiceProviders { get; set; }
    #endregion
}

 public class Departament
{
    #region Attributs
    public int DepartamentId { get; set; }
    public string Description { get; set; }
    public DateTime DateRegister { get; set; }
    public DateTime DateModified { get; set; }
    #endregion
    #region Relationships Other Class
    public virtual IEnumerable<ServiceProvider> ServiceProviders { get; set; }
    #endregion
}

In the ViewModel (Presentation Layer)

public class ServiceProviderController : Controller
{
    private readonly IServiceProviderAppService _serviceProviderApp;
    private readonly IPositionAppService _positionApp;
    private readonly IDepartamentAppService _departamentApp;

    public ServiceProviderController(IServiceProviderAppService serviceProviderApp, IPositionAppService positionApp, IDepartamentAppService departamentApp)
    {
        _serviceProviderApp = serviceProviderApp;
        _positionApp = positionApp;
        _departamentApp = departamentApp;
    }

    // GET: ServiceProvider
    public ActionResult Index()
    {
        var serviceProviderViewModel = Mapper.Map<IEnumerable<ServiceProvider>, IEnumerable<ServiceProviderViewModel>>(_serviceProviderApp.GetAll());
        return View(serviceProviderViewModel);
    }

    // GET: ServiceProvider/Details/5
    public ActionResult Details(int id)
    {
        return View();
    }

    // GET: ServiceProvider/Create
    public ActionResult Create()
    {
        ViewBag.Position = new SelectList(_positionApp.GetAll(), "PositionId", "Description");
        ViewBag.Departament = new SelectList(_departamentApp.GetAll(), "DepartamentId", "Description");
        return View();
    }

    // POST: ServiceProvider/Create
    [HttpPost]
    public ActionResult Create(ServiceProviderViewModel serviceProvider)
    {
        if (ModelState.IsValid)
        {
            var serviceProviderDomain = Mapper.Map<ServiceProviderViewModel, ServiceProvider>(serviceProvider);
            _serviceProviderApp.Add(serviceProviderDomain);
            _serviceProviderApp.Save();

            return RedirectToAction("Index");
        }

        return View(serviceProvider);
    }
}

In View:

<div class="form-group">
            @Html.LabelFor(model => model.DepartamentId, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.DepartamentId, (IEnumerable<SelectListItem>)ViewBag.Departament, string.Empty)
                @Html.ValidationMessageFor(model => model.DepartamentId, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.PositionId, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.PositionId, (IEnumerable<SelectListItem>)ViewBag.Position, string.Empty)
                @Html.ValidationMessageFor(model => model.PositionId, "", new { @class = "text-danger" })
            </div>
        </div>

The GetAll () and CRUD methods are called by generic repositories. The problem is exactly when I try to submit the form. Displays the error I mentioned earlier.