tried deleting an OPEN invoice but getting this error - > YOU_CANNOT_DELETE_THIS_TRANSACTION_BECAUSE_THERE_ARE_OTHER_TRANSACTIONS_LINKED_TO_THIS_ONE. i created a transaction and looked for any transactions linking and applied (Applied To & Applying Transaction) to the invoice but found nothing.

look under related records tab for hints.

there are no related records

I would submit a support ticket with NS then. interesting.

It happened to me once,, it was not an invoice, but I could find it on a report.. but if you have a lot of data, it would be a bit difficult, I didn't have much data so I could find it easily since it was the only data there was.

If you have advanced revenue recognition or suitebilling modules, some of the related records won't appear on the subtab.

Check for any customer deposit application or Credit Memo linked to Invoice/SO