Why is 'We do not have a bed to sleep in' wrong? I can't see any plural marker, and, in English, it seems much more natural to say 'a bed' in this case.
I think it should probably be accepted. The problem as with all language translations is that you can not simply swap word for word like a code book. The way of expressing thoughts in different languages will usually have many possible variations and this particular course has not yet evolved enough to include more than a few. Don't worry about it and move on. It will no doubt get better over time.
I would think this is correct and makes more sense : we don't have a bed to sleep on. As in (my wife and I) We don't have a bed to sleep on.... :)