How do I set purchase price to vendor sublist on item record while running in schedule workflow and workflow action script?

I used below sample code but it's not setting the purchase price to vendor sublist record. But I try to load the record and save in Workflow action script it's working as expected. However

says that "you can't use record.Record.save()".

var VENDOR_ID = '1954';
var VENDOR_CURRENCY_ID = '1';
var PURCHASE_PRICE = '4.5';

function onAction(context) {
  var newRecord = context.newRecord;
  var recordType = newRecord.type;
  var recordId = newRecord.id;

  // When we try to load and save the record then it works as expected and set the purchase price to vendor sublist.
  // var itemRecord = record.load({
  //   type: recordType,
  //   id: recordId
  // });

  // var vendorSublistLineCount = itemRecord.getLineCount({ sublistId: 'itemvendor' });
  var vendorSublistLineCount = newRecord.getLineCount({ sublistId: 'itemvendor' });

  log.debug({
    title: 'vendorSublistLineCount',
    details: