Try using @Html.DropDownList instead DropDownListfor
You cannot used that Helper @Html.DropdownListFor, because the first parameters was not correct change your helper to:
@Html.DropDownList("accountid", new SelectList(ViewBag.Accounts, "AccountID", "AccountName"))
@Html.DropDownListFor receive in the first parameters a lambda expression in all overloads and is used to create strongly typed dropdowns
If your View it's strongly typed to some Model you may change your code using a helper to created a strongly typed dropdownlist, something like
@Html.DropDownListFor(x => x.accountId, new SelectList(ViewBag.Accounts, "AccountID", "AccountName"))
Here is Documentation
;_)