Please mention Which trains are you trying to reserve? Day/Route?
And where you tried it?

Otherwise we can just guess as there are several options why you can´t reserve.
From train sold out to Booking system not working all could happen

Hi, thansk for your answer.

I am trying to get a reservation for the TGV from Munich to Paris (6th of August) leaving at 6:46 in the morning in munich main station. (TGV 9576)

Another train is from Paris to St Malo on the 9th of August leaving at 10:56 in Paris with a stop in Rennes (TGV 8615 then TER54339)

The last one is from St Malo back to Munich leaving at 11:59 on the 16th of August in St Malo (arriving in Paris at 14:45) (TGV 8084 then TGV 9577)

Thanks :)

M→ P answered above.

For IN FRance there is a small (if at all?) Quota for 10€ extra-if exhausted you can still get any seat for 20 extra (for short trips may be more as normal ticket). The app cannot handle the last ones and then shows what you saw.

Either call SNCF (only worthwhile if you have at least a cr-cd) or be a little flexible (which also means same as trip 1-perhaps change as very few direct TGV to StM) and do it at SNCF machine right after arr in FRance. The last NOT possible for overtheborder-only inland

Did you ask samesame at DSO? where they told you to use the local TER-trains

TGVs between Germany and France can be booked at the DB ticket office or by phone. You can also book them by phone via SNCF. Press #85 for English, no booking fees, reservations are sent via e-mail. TGVs within France can also be booked via SNCF by phone.