Hi, I need a bit more information to be able to help you. What ways did you try to make a reservation and why specifically you can't make the reservation?

It's best to post your exact timetable, so people can give better advice. In general though, long distance trains from France can be booked at the SNCF ticket office (or by phone). You'll probably change in Germany to an ICE train and there a reservation is optional. You can make a reservation in Germany  via https://reiseauskunft.bahn.de//bin/query.exe/e (click on Seat Only).