Copy the cost from custom field to native Est. Unit Cost on Inventory Adjustment

Hello,

We have Group Average Costing enabled. We have also created a custom field to source the Average Cost. This field has been made available on Inventory Adjustment (IA) at the line level. The expectation is to copy the value from this custom field to Est. Unit Cost (native field). Since the native field is not available on workflows, we created a user event script to copy the value. But when saving the IA, we get the 500 error, which I found out by checking the browser console.
Attaching the code here if someone can help where the issue is.

/** * @NApiVersion 2.1 * @NScriptType UserEventScript */define(['N/record', 'N/log'], (record, log) => {        function beforeSubmit(context) {        try {            if (context.type === context.UserEventType.CREATE ||                 context.type === context.UserEventType.EDIT) {                                const newRecord = context.newRecord;                const lineCount = newRecord.getLineCount({ sublistId: 'inventory' });                                log.debug('Script Started', 'Line Count: ' + lineCount);