If the transaction is pending, the money is still techincally in your account, just ring fenced. If you click “Something wrong?” one of the options should be to lift the hold.
The company still could collect the money however and if that does happen, then that’s when you would need to wait 14 days.
If you do not have the option and the payment is still marked as pending, contact Monzo via in app chat and they can manually remove the hold.