This means that all the meeting locations are full. This may just be on one timeslot or, if you have chosen Auto-Select mode when setting up the event, all the timeslots are fully booked. You can add more meeting locations, if venue space and costs permit, or it may be that rescheduling of a meeting request will allow it to be confirmed. 

If the meetings are all taking place on Custom meeting points (delegates' own locations), a delegate will become unavailable when all their meeting timeslots are booked: this also triggers the 'No spaces' alert.