This article covers the most common reasons you cannot select an open slot for your reservation.
Issue: You or a guest are trying to make a reservation and there is only one other resource being used at the time you are making a reservation. You should be able to make a reservation because the time slot is open, so why can't you?
Staff Capacity of Package
Each package has a field within the schedule labeled "staff capacity." This field allows you to set how many other packages can start at the same time as this package, so you can manage how many package starts your staff can handle. If your staff capacity is 1, and 1 other package is starting at 4:00pm. you cannot select 4pm as a start time for this package. You'd need to raise your staff capacity number or select another time.
Your Package is Set to a Longer Duration Than Time Allotted
If your schedule has 2 hours allotted, but your reservation is 4 hours (or in the videos case, 2 hours with a 1 hour buffer = 3 hours total), your reservation is going to show no availability. Your duration has to fit within the time you have allotted. The fix is to open your schedule window to fit your duration, or lower the duration of the package.