ERR_INVOICE_PAID on Xero: Invoice already paid. Root cause: The invoice has already been marked as paid and is now locked against further modifications. Accounting systems lock paid invoices to maintain an accurate audit trail. If a correction is needed, a credit note or adjustment must be issued rather than editing the original invoice. Step 1: Check payment status. Verify invoice paid Step 2: Unpay invoice. Remove payment if needed Step 3: Make changes. Update invoice Step 4: Repay invoice. Mark paid again