Buy ticket-at evidently the normally open national counters-as far as PYCE (Ruse)=last stop in BG, border check point. In fact it is not a train as such, but just 1, maybe 2 cars that get shunted on+on and make a big detour in RO due to some long-standing blockage.

Assume from your text youre not on a pass? In that case- RES is not really needed.

But notice is useful for future people who get the advice to wait till SOFia for going where you cam from

Thanks for the reply but we are on an interrail pass and according to the app we need a reservation for any part of the journey.

You can always ask the coach attendant if the ticket office is closed.

There’s no need for a reservation for the Sofia-Bucharest train. 
Sometimes Romanian conductor demand one, but according to Bulgarian staff it is not neccessary and I used it several times without a reservation.

Thanks for the answers, spoke to staff on the platform and they told me just to board so happily in Bucharest now.

Neither Bulgarian nor Romanian railways have marked in the timetable data they send to the European Timetable Centre that this train/these trains have compulsory reservation (see e.g. DB, ÖBB, CD planners). That also means that the rail planner app is wrong.