If you need to cancel a bank transaction that was included in a statement that has been reconciled, then you must unreconcile the bank statement first from the Bank & Cash menu.
Select Bank & Cash > Unreconcile a Bank Statement.
Select the statement you want to unreconcile.
The transactions in the bank statement are unreconciled and you can now cance the bank transaction.
Note: Unreconciling a statement will cause all subsequent reconciliations to be automatically unreconciled. So if in June you unreconcile a statement from April, then statements from May and June will also be unreconciled.