Upvote:1
It is very likely a queued system, first in, first allotted. With only a certain number of appointments available on any given day, it would mean that all were taken before your 'request' hit the server.
Anecdotal feedback suggests that this is not uncommon with the Prenota online appointment system. Apparently, about 10-12 appointments are released per day, so it may take some persistance, timing and tactics (e.g., multiple concurrent browsers).
As example, Prenota New York suggests that
In order to book an appointment with this visa office, we urge all our applicants, who could not find any availability, to repeatedly revisit our online appointment system since cancellations are quite regular. Please, be aware that the best time to check for appointment availability is after 6pm on.