A transaction can be locked for the following reasons:
1. A payment has been applied to an invoice you are trying to delete. You must must delete the payment and then you can change or delete the invoice.
2. The VAT period pertaining to the transaction has been closed. You must first reopen the VAT period (Tools, Locks) to change or delete the transaction.
3. The Bank account has already been reconciled for this transaction. You must first reopen the bank rec (Tools, Locks) to change or delete the transaction.