public class ReservationsController : Controller
{
    private AppDbContext db = new AppDbContext();

    // GET: Reservations
    public ActionResult Index()
    {
        return View(db.Reservations.ToList());
    }

    // GET: Reservations/Create
    public ActionResult Create()
    {
        ViewBag.ScreeningId = new SelectList(db.Screenings, "Id", "Description");

        HashSet<int> seats = new HashSet<int>(db.Reservations.Select(x => x.SeatNumber));
        ViewBag.Seats = seats;
        return View();
    }

<div class="form-group">
    @Html.LabelFor(model => model.Screening, "Seans", htmlAttributes: new { @class = "control-label col-md-2" })
    <div class="col-md-10">
        @Html.DropDownList("ScreeningId", null, htmlAttributes: new { @class = "form-control" })
        @Html.ValidationMessageFor(model => model.Screening, "", new { @class = "text-danger" })
    </div>
</div>

'There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key 'ScreeningId'.'