You are only able to add instructors to an event if they have been given the permission from AYSO Head Office.
If the Instructor does have the permission to assess the course within the event please ensure that they have logged into the learner account and accepted the terms and conditions. They will also need to switch into their Instructor account and accept the terms and conditions here.
Once they have logged in you should then be able to add the instructor to the event.
If this is still not working please contact the AYSO Head Office to confirm their permissions.