If you are unable to complete a booking to a Stallion, the horse may be missing some required settings.
Perform these checks for that Stallion in your Horses list;
Navigate to Main > Horses
Perform a search for the horse in question
Double-click to open the horse details screen
Ensure Current Horse is ticked
Set Type to Stallion
Tick the option Standing at Stud
Click Save/update
With the above checks in place, try to create the booking.
For more information, refer to the Ardex Premier User Guide - Horse Breeding with Ardex.