I believe "not possible" ONLY refers to the listing no longer being available some of the days of the request.
It is possible for guests to book multiple rooms at the same time even though this sort of violates airbnb policy (the person booking the room is expected to stay so if 2 people need 2 rooms they technically should each make their own booking--however it isn't implemented to enforce this).
So check your calendar and you'll see that there's a day booked during that reservation period.
In such cases, I'll usually make a special offer for a different room if I have one available and hope to get the booking.