All group bookings must have at least a soft or a hard allocation for at least one room type for each day of the group. Room Type allocation is set up during the group booking process in the Group Booking Wizard, but can be edited at any time in the Edit Tab of the Group Booking.
Because a group booking isn't allowed to overbook a property you can find a scenario where the type has been overbooked by staff and when editing the current availability on the type shows –X and you get an error "No. of rooms allocated exceeds available no. of rooms for the selected room type(s) for at least one of the days" To better understand how this can happen and how to resolve it please see the following Knowledge Base article. Group Bookings: Rooms Allocated Exceed Available Rooms
if you cancel a booking in a Group, the Room Type allocation is still in effect and will hold availability. You may see availability #'s on the Tape Chart like -1. To release Availability, change your hard allocations in the Edit Tab of the Group Booking. See Bookings: Resolving Tape Chart Availability Issues