On this route it is not possible to get a reservation online via DB. Only at their Ticket offices, Call center or SNCF call center or ticket office. Online only via Interrail.eu

But Interrail.eu send then a paper reservation :/

Thank you a lot for the help! Do you think I can wait to buy the seat reservation just before I get on the train?

The best way to make reservations for French trains is to call SNCF. No need of Pass Cover Number. 

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English, no booking fees, reservations are sent via e-mail

In this case also possible via DB Call Center, but there you have to collect the reservation at a ticket machine in Germany or via postal mail. 

Is that new? That used to be an online reservation.

Depends on the date, but in general better to make it in advance… As you are traveling during summer season, it might be sold out otherwise :/

IF full-use local train to reach Strasbourg via Offenburg and then TGV (ev hour) domestic FR to P=much cheaper too, but takes more time and have to compete with the overlaod on 9€ tix in DE.

These fast trains IN (and thus also TO) FR have special small quota for passholders, so fior pass may appear ´full´ but not for higher payers.

A friend got it as paper reservation with CIV 1080 which means they booked via DB bookingsystem :)