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
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.