There is no viewdata item of type 'ienumerable<selectlistitem>' that has the key 'dept.departmentid'.

namespace BusinessLayer
{
    public class Workshop_BL
    {
        [Key]
        public  int RecID { get; set; }
        public  string AssetNo { get; set; }
        public Departments Dept { get; set; }
        public  string Location { get; set; }
        public  decimal Cost { get; set; }
        public  string Description { get; set; }
        public  DateTime WoDate { get; set; }
        public  string WoNo { get; set; }
        public  DateTime? ExpDate { get; set; }
        public  DateTime InputDate { get; set; }
        public  string WStatus { get; set; }
        public  string Remarks { get; set; }
        public  int? Manpower { get; set; }
        public  int? EstDays { get; set; }
        public  int? EstHours { get; set; }
        public  DateTime?  PartsRecvDate { get; set; }

    }

    public class Departments
    {
        [Key]
        public int DepartmentID { get; set; }
        public string DepratmentName { get; set; }
    }


}

public IEnumerable<Departments> Dept
{
    get
    {
        List<Departments> deptlist = new List<Departments>();
        DataTable dt = DA.GetData("SP_GETALL_DEPARTMENTS");
        foreach(DataRow dr in dt.Rows)
        {
            Departments dpl = new Departments();
            dpl.DepartmentID = Convert.ToInt32(dr["DepartmentID"].ToString());
            dpl.DepratmentName = dr["Department_Name"].ToString();
            deptlist.Add(dpl);
        }

        return deptlist;
    }
}

[HttpGet]
public ActionResult Create()
{
    WorkshopActivity_Bl objWs = new WorkshopActivity_Bl();
    List<Departments> dpl = objWs.Dept.ToList();
    IEnumerable<SelectListItem> items = objWs.Dept.Select(c => new SelectListItem { Value = c.DepartmentID.ToString(), Text = c.DepratmentName });
    ViewBag.Dept = items;
    return View("Create");
}

[HttpPost]
public ActionResult Create(Workshop_BL bl)
{
    if (ModelState.IsValid)
    {
        WorkshopActivity_Bl objwl = new WorkshopActivity_Bl();
        objwl.InsertWorkshopActivity(bl);
        return RedirectToAction("DetailsList");
    }
    return View("Create");
}

        <div class="editor-label">
            @Html.LabelFor(Model => Model.Dept)
</div>
        <div class="editor-field">
                        @Html.DropDownListFor(Model => Model.Dept, (IEnumerable<SelectListItem>)ViewBag.Dept, "Select")
                        @Html.ValidationMessageFor(Model => Model.Dept)
        </div>