Update assignment error

When I update a assignment,the code:

                StringBuilder changeXml = new StringBuilder();
                changeXml.AppendFormat("<Changes><Proj ID=\"{0}\">", projectUid.ToString());
                changeXml.AppendFormat("<Assn ID=\"{0}\">", assn.ToString());
                string pidPctWorkComplete = PSLibrary.AssnConstID.s_apid_pct_wrk_complete.ToString(System.Globalization.CultureInfo.InvariantCulture);
                string pidOvertimeWork = PSLibrary.AssnConstID.s_apid_ovt_work.ToString(System.Globalization.CultureInfo.InvariantCulture);
                changeXml.AppendFormat("<Change PID=\"{0}\">{1}</Change>", pidPctWorkComplete, "50");
                changeXml.AppendFormat("<Change PID=\"{0}\">{1}</Change>", pidOvertimeWork, 60 * 10000 * 8);// 8 hours
                changeXml.Append("</Assn></Proj></Changes>");
                Console.WriteLine("Updating Status");
                #endregion
                #region Update Status
                statusingSvc.UpdateStatus(changeXml.ToString());

But it's tell you StatusingPidNotEditable.what's wrong?