Hello,
I believe that seats should be available for reservation in December as the trainline has been opening schedules.
At the moment Italiarail is not offering railpass services, only point-to-point tickets. You can book seat reservations on https://www.interrail.eu/en31, https://www.raileurope.com/en/destinations/passes/eurail-global-pass43, or https://www.eurail.com/en32.
If you are having trouble booking I would recommend reaching out to Eurail directly.