It's a night train, with seats for 3€ but also with couchettes and sleepers for higher prices. It may also be that the seats are already fully booked. I can't check right now; the reservations module has stopped working suddenly. It'll probably be back tomorrow.

Are you currently in Italy? Then you can also go to a ticket office.

Yeah the reservations have been really buggy today. I just need the seat. 

I’m not in Italy right now. I know I can go to the station to buy it, but I’ve never used Eurail. Is there a chance they sell out beforehand? I won’t be at the station until that day so it’s pretty important I’m on this specific train.

It's always possible that a train is fully booked. I checked on the Trenitalia website and it only offers couchettes and sleepers, no seats. That's why the price weent up when you tried to book. The lowest normal fare available is currently €87.90. That's about the price of the Eurail supplement and you don't need to use a travel day. (Yes, Italian night train supplements can be more expensive than an advance ticket and you can currently only book a complete compartment - still a COVID measure.)

@Nanja, this is an error in the app: there are no seats on this train. The DB timetable also says only couchettes and sleepers. So the app shouldn't propose 3€ seat reservations.

So do you recommend just spending the money and booking beforehand rather than showing up the day of and risking it being sold out?

I don't know how busy that train is usually and what the chance is to find free places. But if catching it is important to you, then you should book in advance. But remember: there are no seats on that train and if the cheapest pass reservation is over 100€, then a normal ticket is currently cheaper than that.

Rvdborgt, to come back to your comment, the indication prices in the App and website are based on the train type, not a specific route. And because on other Italian night trains the second class seats are available, the indication prices exist. It is not possible to manage this per route. 

The prices are indications to get a general idea. For real prices we have the Reservations Tool. 

This is quite unsatisfactory. It means that, depending on the route, incorrect information is supplied, e.g. certain categories shown, such as seats, may never exist in the train shown. You should really plan an enhancement to manage this per route. In any case, there should be a warning that certain categories may not exist in the train shown, in addition to the warning about availability.

Thanks for your comment, I've passed on your feedback to the reservation team.