I'm sorry, but that is the schedule we will have to stand by?

One of our client sent us a RFP. But unfortunately our proposal Team is busy and we can't make the deadline. I informed this to the client and he replied as 'I'm sorry, but that is the schedule we will have to stand by.' I know that it's our fault and I wanted to reply him. Can any one help me to frame reply to his reply 'I'm sorry, but that is the schedule we will have to stand by.'