Any transactions in a VAT period marked as filed can’t be changed on the Flat Rate Scheme

I totally understand your frustration, especially when you spotted the mistake straight away. To fix this, you’ll need to Duplicate the invoice again with the correct date and then reverse and re-enter the transaction, as QuickBooks didn’t allow to edit or delete the included transaction once the VAT is submitted. 

Here’s how:

1. Open the invoice with the wrong date.
2. Click More Action at the bottom, then choose Duplicate.
3. On the new copy, set today's date, check the quantities and invoice number, then select Save.
4. Open the Transaction journal for the original entry.
5. Create a journal entry with reversed debit and credit amounts from the original transaction. Use the same accounts.
6. Apply the relevant VAT code (e.g. 20.0% S) to the same line item.
7. Create a new transaction with the corrected information.

Since the message you received is working as designed, it acts as a digital seal on your historical accounting records. 

On the other hand, you can share your feedback with our engineers since you'd like to see a proactive warning prompt added when duplicating transactions across closed periods. This will help them understand your needs and consider your feature request for future updates.

Please leave a reply below if you have any other concerns.