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.