Error creating an ItemFulfillment

I've made an attempt to create an ItemFulfillmentPackageList and ItemFulfillment via a Suitelet, but received the following error message.

Error: INVALID_RCRD_TYPE
The record type [ITEMFULFILLMENTPACKAGELIST] is invalid.


Here is the code snippet I used to create the records:

  var filters = new Array();
  filters[0] = new nlobjSearchFilter('tranId', null, 'is', '123');
 
  var columns = new Array();
  columns[0] = new nlobjSearchColumn( 'internalId' );
  columns[1] = new nlobjSearchColumn( 'entity' );
 
  var searchresults = nlapiSearchRecord( 'salesorder', null, filters, columns );
  if (searchresults != null && searchresults != '') {
   // We found an internal ID of the sales order, so this record must exist.
   var orderInternalId = searchresults[0].getValue("internalId");
   //var order = nlapiLoadRecord("salesorder", orderInternalId);