You are using dynamic types in extension methods, which is not supported.
Cast the dynamic types to actual types, and it will work.
From what I see now, I’d say:
(string) ViewBag.MagNo
Which would result in
@foreach (var item in Model)
{
@Html.DropDownListFor(modelItem => item.TitleIds,
new SelectList(ViewBag.TitleNames as System.Collections.IEnumerable,
"TitleId", "Title.TitleText"),
"No: " + (string) ViewBag.MagNo,
new { id = "TitleIds" })
}