Which trains are you trying to make reservation for? Usually there are easier places to do it than through Interrail.

There is good guide to how and where to make reservations here

and also on the excellent Seat61 site

https://www.seat61.com/interrail-and-eurail-reservations.htm

For example the IC103 between katowice and Wien on the 16/06/2023 

thank you for the links

Easy to to on the ÖBB website for €3, but reservations for that date aren’t open yet, you'll need to wait. If you put in a date next week, for example, you’ll see the availability.

You can book a reservation on tickets.oebb.at with the "Seat reservation only” option, for €3. Reservations are open 60 days in advance for trains from/to Poland.