Have a look at these pages :

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

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Spanish seat reservations are best done at a RENFE ticket counter, see : https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105/index2.html?postid=18419#post18419

If you have further questions, mention the route, date and departure time.

Hola estoy en Argentina y quiero hacer una reserva de asientos para mi viaje próximo de Berlín a Niza y no puedo hacerlo desde la página oficial

Será porque aun No estoy en Alemania?

Hola estoy en Argentina y quiero hacer una reserva de asientos para mi viaje próximo de Berlín a Niza y no puedo hacerlo desde la página oficial

Será porque aun No estoy en Alemania?

Which route will you take? Reservations are optional in Germany, not possible in Switzerland. In Italy and France they are compulsory for high speed and Intercity trains.

You can buy German and Italian reservations, and also the cross-border Swiss>Italy on ÖBB

https://shop.oebbtickets.at/en/ticket

French domestic reservations here

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

If you're travelling after December 10, you may need to wait for the new timetable to be published in a couple of weeks.