Which train and travel date ? Interrail is not the best place to book : 2€ fee per person per train and more expensive optional seat reservations.

Eurocity : 3€ optional seat reservation on ÖBB.

All other trains : seat reservation not possible.

Hello. I should book seats on the train to get from rome to Ljubljana, but I can't find any site that offers me this route. 

There are no direct trains. I guess you'd like to take this connection :

- FR Roma Termini - Venezia Mestre 05:35 - 09:23 13€ on ÖBB

- RE Venezia Mestre - Trieste Centrale 09:51 - 11:44

- EC Trieste Centrale - Ljubljana 12:52 - 15:40 3€ optional seat reservation on ÖBB

To book on ÖBB : add Interrail as a discount and select one-way tickets.

There is information on the service at Seat61

https://www.seat61.com/trains-and-routes/venice-to-ljubljana-and-zagreb-by-train.htm

Editted to remove info that may be misleading

Hope it helps.

EDIT: I stand by Seat61 as a resource, but I’d go with the more experienced thibcabe and use OBB for reservations. Seat61 does link to it down the page.

Bahn.com is a good planner but it cannot book seats outside Germany. ÖBB is the website to use : works in Italy, Germany, Austria, Czechia, Hungary,...

Unfortunately, all the sites you suggested do not have a schedule that includes a direct route between Trieste and Ljubljana, and I do not understand how I can book this short route. If for Rome-Venice I found a solution through Italiarail and the venice triesto route I can do it without a seat reservation, unfortunately I can't figure out how to book a seat for the trieste lubiana route. If you can point me to an alternative site to interrail.eu.

There is definitely a train direct between Trieste and Ljubljana. One leaving at 12.52 is no reservation. I just used a random date.

How far in advance are you trying to book for? If it is too far in advance there may not be Italian timetables for that date. That is something I came across when looking for summer trains in Italy back in the spring.

I gave you the link to ÖBB. This website works, is reliable and has no booking fee (unlike ItaliaRail or Interrail). Add Interrail as a discount and select one-way tickets.

Search legs separately : first Roma Termini - Venezia Mestre and then Trieste - Ljubljana.

You are not required to book Trieste - Ljubljana. Even without a seat reservation you'll certainly get a seat.

There are not many trains between the two cities : 3 daily trains.