There is no report but try this. Just an idea if you have to go back through the appointment book to find what columns had open slots. This view matches your appointment book just no patient names appear.
Click on this icon Show overview.
Click on the arrows to take you back a week.
Click this icon to increase or decrease the size of the appts. Less is more. 1.5 is great but in case your monitor is larger.
White is the appointment and yellow is an open slot.