Where are you trying to book seats? Rail planner app, Interrail website, Italiarail.com, another place?

For which journey? Departure/arrival stations, date, time?

I tried rail planner app and iterrail website, I need 3 resevations. 4 July-Bologna to Rome (8:37-10:40), 6 July Roma to La Spezia (9:57-14:03) and 9 July Florence to Venice (8:20-10:38)

trenitalia.com indeed shows enough seats left on these trains. There could be a temporary problem connecting to the Trenitalia reservation system.

Alternatively, assuming you're still at home, you could try to book at a Czech Railways ticket office. That could even be cheaper, as the Interrail website and app charge an extra fee of 2 EUR per seat.

Ok, I will try that. Thank you very much!

I’m experiencing same problem for seat reservations for my trip to Italy, maybe there is some problem system, but I’m planning to start my travel in few days and this makes me very nervous. Please if you see that problem is fixed, let me know.

Do you have a station with an international ticket office near you? Then you could ask there. For the rest, it's usually no problem to make reservations a few days in advance at an Italian ticket office.

There is train station in Rijeka where is possible to buy some international tickets, I will try to ask there tomorrow, maybe they will be able to help me, but since in Croatia railway connections are bad, I’m not sure if they are informed enough about those things and able to sell reservations for Interrail passes.

Well, if they can sell tickets for Italian trains, then there's a chance they can also make Interrail reservations for Italian trains. The best is to write down (or print) the exact trains you want to use: departure station/date/time, arrival station/date/time and train number. That usually helps.

I went to train station in Rijeka today and they made reservation for me. It seems like it is possible to make Interrail reservation in any ticket office, that is good to know and it is even cheaper, no reservation fee of 2 euros.

It can be done in many ticket offices that sell international reservations. The number of such ticket offices varies from country to country.

Hi, I am sorry to hear you have been experiencing issues when making reservations. This is a bug we are aware of and are trying to fix. As a temporary workaround, you will need to go to the '’My Trips & Travelers section, edit your travelers and add the missing data again. In case you still experience issues, please contact our Customer Support team directly via the support options listed here: https://www.interrail.eu/nl/support

You can also opt for any of the alternative ways of booking that have already been mentioned. Cheers 

Still getting the error about leg of the journey not being available..I followed the workaround about “My Trips & Travelers” but there is no data missing there/everything seems to be accurate. I haven’t heard anything back from support. Can you provide us with a solution so we can book Milan (centrale) to VISP?

If you're currently in Italy, go to the ticket office. Also in neighbouring countries, the ticket offices (the ones that can sell international reservations, usually at larger stations) would be able to sell this reservation.