getSelectValue returns INSUFFICIENT_PERMISSSION

Hello,

I am trying to get the list of items from a custom list field. Here is the Code:

                GetSelectValueField gsvf  = new GetSelectValueField();

                gsvf.fieldType            = GetSelectValueType.customList_customValueList_valueId;
                gsvf.fieldTypeSpecified   = true;
                gsvf.searchCriteria       = ConfigurationManager.AppSettings["VendorServicesListId"];

                getSelectValueResult gsvr = _WebService.getSelectValue(gsvf);

The status detail in gsvr tells me INSUFFICIENT_PERMISSION, and the message is:
"You do not have permissions to set a value for element valueId due to one of the following reasons: 1) The field is read-only; 2) An associated feature is disabled; 3) The field is available either when a record is created or updated, but not in both cases."