If the room booking system does not offer you a suitable location on the day and time that you require, it means that all suitable locations have already been booked out.  You should ensure that you have correctly inputted all the fields on the room booking request form, and have not inflated the number of attendees and are not requesting anything that you do not require e.g. tables or a large space for 60 when there are only 20 people expected to attend the event.