Use tickets.oebb.at : add Eurail as a discount and select one-way tickets. Seat reservations cost 3€

The Austrian Railways website works in many other countries and has no booking fee.

If it doesn't work, let me know your travel date

ya pude comprar la reserva de varsovia a viena pero tuve que elegir otro viaje porque cambian en comparacion con la app de eurail

lo que no pude comprar fue reserva  para cracovia-varsovia la web polaca me hace generar un usuario y despues no se activa

Eurail Rail Planner is not reliable. OBB is connected to the European-wide database so it’ll show the correct times. If the schedule hasn’t been updated on Rail Planner until then, add the train manually.

For trains within Poland have a look here : https://www.seat61.com/interrail-and-eurail-reservations.htm#Poland

Seat reservations cost 0.25€ and cannot sell out.