In Romania, all long distance trains require a reservation. They're not expensive (IR €1 or IC €3.60), and you can usually still get them last minute but you do need them.
In Hungary, you also need an inexpensive seat reservation in most long-distance trains. In Slovakia only in 1st class.
See this page where best to book reservations:
https://www.seat61.com/interrail-and-eurail-reservations.htm
That's usually not via Eurail. They charge an extra €2 booking fee per person and train and that quickly adds up. Also note that the app can't book anything, it only forwards to some of the websites where you can book, including the Eurail website.
Finally, don't plan using the rail planner app or the Eurail website. They're incomplete, often out of date and not reliable. National or the operators’ websites/apps are much more reliable; for international journeys, bahn.com or the DB Navigator app is a good choice.
Thanks for info, I'll follow that link and do my research , not throwing Euros away for no good reason.
3 euros at Bucharest to Budapest at Int Ticket office compared to 12 euro on the App