When One World NetSuite users edit the item record to select a subsidiary, the selected subsidiaries become deselected from the list, hence generating the error 'You cannot remove subsidiary: XXX because this record is used on a transaction for subsidiary: XXX'
Here is a quick way to solve this error:
1. Edit the Item record.
2. Locate the Subsidiary section.
3. Press CTRL key on the keyboard (Windows) or Command/Apple key (Mac).
4. Select the Subsidiary to be added.
Note: Pressing the CTRL key will not deselect the previously selected subsidiaries as this is multi-select field.