I can see there is availability but why am I unable to make a reservation?

First you click the unit type on the grid, to see if there are any conflicts. If there is a conflict, un-assign the unit(s) in question, also be sure to check for any pencil-ins that may have been created on the unit in question, the system should sort the conflict, but if it can’t you may need to cancel a booking in order to fix the issue. In future it would be advised to manage inventory at peak periods.