You can only delete or edit invoices when they are not paid and are not included on a VAT Return.
If you have a payment or credit note attached to the invoice, you'll need to delete them before you're able to delete the invoice.
Once deleted the invoice status shows as Void and you won't be able to recover the invoice. For auditing purposes, you can't re-use the invoice number.
Delete an invoice included on a draft VAT Return >
If you've only created the VAT Return and not yet submitted to HMRC, delete the draft VAT Return first.
You can now delete the invoice.
Delete an invoice included on a submitted VAT Return >
Once you've included an invoice on a VAT Return you can't edit or delete it. Reverse the invoice by creating a credit note.
The credit note will appear on the next VAT Return, and your VAT liability adjusted.
NOTE: On the cash accounting scheme, it's the payment, not the invoice, included on the VAT Return.
NOTE: Once you delete/void an invoice, you can't get it back and would need to enter a new invoice instead.
TIP: If you need to see what was on the voided invoice, you can view it within the Audit Trail report.