So to parry as a Swashbuckler, you need to expend a use of an Attack of Opportunity. Normally, this means you cannot parry while flat footed. However if you have Combat Reflexes which allows you to do Attacks of Opportunity while flat footed, can you then Parry while flat footed then?
You can parry when flat footed, because nothing says you can’t perform that action while flat footed. But you wouldn’t be able to reposte since it requires an immediate action, which being flat footed prevents.