The AZ site only seems to work for revenue tickets issued by the website.
As you have the
ticket number, look this up (the ticket number, NOT the PNR/reference) on the "Manage my Booking" tab of the
Saudi Airlines website. The first 3 digits - which must be 055 for an Alitalia ticket - go in the small box; the remaining 10 digits of the eticket number should go in the main box.