It’s not good to decline booking requests, but as far as I know, there is no penalty per se, as long as you don’t make a habit of it. It might affect your search placement if it is perceived that you aren’t a serious host. It’s important, though, that you do reply with an explanation within 24 hours. Not doing so will affect your response rate statistics, which is published on your listing.
Also, there is a difference between an inquiry and a booking request. As long as you respond to an inquiry in a timely manner, you don’t have to do anything further, e.g. you don’t have to accept or decline. I’m not sure which one you got, but just keep that difference in mind.