I'm afraid that all the flights of September 12th are booked. 很抱歉，9月12日的机位已满了。

How about September 13th? 那9月13日的呢?

It's fully booked at the moment. 现在都订满了。

I'm very sorry, but the flight is completely booked. 实在抱歉，那趟班机全部订满了。

I can put you on a waiting list if you would like. 如果你愿意，我可以把你排在等候名单上。

I'll just check to see if there have been any cancellation. 我会来查一下是否有取消预订的。