Some reservations are only available on paper, unfortunately. Sometimes, there are alternatives. If you say which trains you would like to book (origin, destination, date, departure time), then people can suggest other ways of booking.

This has been explained here a few 100 times. AND you do not need to make any REServ via the site/app-there are numerous other ways, depending on what railway/sector.

The culprit is most often the INTERnat TGV trips to/fro FRance-these can also be done by SBB at counter, if you are in CH now.

You have to think the way that all railways ran their things untill not that long ago: for these things you go to a station and tell what you want, show pass, and they do the work. It most often does not even have to be the railway that you book for-they coöperate INTernational since over a century. And you passby/wait in station quite a few times if you travel with pass!

Thank you for your answers. 

I want to travel on the 22. April from Lyon Part dieu (14:35) to Barcelona Sants (19:32).

OK, reservations to Barcelona are only available on paper. They're available in Spain, France and Germany from the ticket office. Not sure about Switzerland, you could ask at the station. If you ask there, please let us know what you find out.

Deutsche Bahn also sell them by phone (+49 30 2970, collect at DB ticket machine or send via the post for €5.90).

The number of pass holder seats is also limited, so you'd better make the reservation ASAP.

Thank you very much. It was possible at the station in St.Gallen Switzerland to buy the seat reservation.

Thank your for your help!

Good to know that SBB also still sell these reservations!