You can call SNCF to make a reservation, see link.
Or else book at an international counter in a station ...

With interrail it doesn't work anymore because the departure date is too close to the current date, I think.

https://www.sncf.com/fr/service-client/nous-contacter/telephone

I have already tried to call several times even when opening but each time it says that all lines are busy and it hangs up after a few minutes.  Isn't there a way to do it directly on the SNCF website?

Hi Mathéo,

on the SNCF website it is not possible.
I am not sure if a ticket office in Slovenia sells this reservation, but you may try your luck in Ljubljana/Maribor.
Otherwise try in Austria or Germany on your way back if you have some chance. In Germany it is for sure possible (if the ticket office is open at the time). In Austria I am not sure, as with the new ticketing system they sell almost nothing abroad anymore… unfortunately.

No, I myself have no experience with calling the reservation number of sncf. But there's no other option, if I'm not mistaken.
Either call Scnf or book your ticket at an international counter in a station.

@Mathéo

You can also call DB Bahn.
I think you will also be able to book this train through them...

Please use the contact details below if you have any questions about the following topics:

International bookings on bahn.de or bahn.com
International travel connections
Other issues relating to international rail travel
DB's service number +49 30 2970; (costs depend on provider), or use our contact form.

Yes I have already been to Ljubljana station to ask but they cannot book this ticket for me.

Thank you for your answers and your efficiency, but in the end I managed to find someone in France who was able to go to the station and buy the seat for me so this problem is half resolved for the next ones.

Thank you for your answers and your efficiency, but in the end I managed to find someone in France who was able to go to the station and buy the seat for me so this problem is half resolved for the next ones.

Is your ticket already reserved?

If so, how much did you have to pay?

@Sven  Here a general overview of TGV Reservations TGV Reservations the reservations from to Germany have a fixed price :)

I'm only seeing this post now

It is an interesting link, although the information is no longer up to date. So I see that the price for the TGV Luxembourg - Marseille is 10 euros and no longer 6 euros.

Unfortunately, this train is not a real alternative to avoid the expensive surcharges when you depart from Brussels.

No surprise unfortunately. In August they couldn't even sell a reservation for a train in Croatia.