Thats the “error” message.

You can book at a DB Reisezentrum your reservation for this train. There are only a few seats for Interrail users in the train, it could also be full. 

I think this is the problem:

It seems there are engineering works and the final timetable is not ready yet. I'd wait until the final timetable is there. Alternatively, you can book it from Mannheim now (reservation is not required in Germany so that's not a problem), but then you're not sure what the changes will be and what that would mean for your connecting train from Dresden.

The best is to book it at the DB ticket office anyway, since Interrail charges an extra fee. The same is true for a reservation Dresden - Frankfurt (optional): also do it via DB. Interrail charges twice as much (8 instead of 4 EUR).

Thanks for your answers. We have found the possibility to book from Mannheim too but we were unsure if we need a reservation in Germany too because it’s an TGV. We would like to book the reservations asap because we need to book other things in france were we must be certain to get to it on time 

We were also at an DB ticket office but they told us that they can’t book TGV but maybe the woman didn't want to help us because it was not far from the end of the workday. 

And there is the problem that i can’t book only the reservation at the DB website (which is in general possible) just the complete ticket.

Reservation in the TGV is not required in Germany. It's only mandatory from the last stop in Germany (here: Saarbrücken).

The Interrail reservations from Germany to France cannot be booked on the DB website, only at the DB ticket office, DB travel agents or by phone.

If you're happy with reservations from Mannheim, then you can book now. Otherwise, you'll have to wait until the timetable has been finalised.

Thanks for your quick answer. I will contact DB directly. 

Thanks a lot ^^

IF you board this train in FRA then just take the seats you have hopefully REServed from SB. They will be labelled as such-you must have used more DB trains, I guess. DD-F is optional RESvation, but it may in the end be necessary to use an earlier ICE as it may be possible train times change due to Werke. AND the record of DB/ICE for ontime is abysmal bad- if it means missing this officially 20 mins connection you have really problems!

Are you really sure that we don’t need reservation in TGV in Germany? Can you confirm this information?  Not that I don’t trust your expertise but I want to make sure that we won’t get kicked out of the train. 

Booking via DB for TGV seems no longer possible or you have to talk to someone that’s really motivated. The woman I was calling this morning told me, seat reservation (only seat not ticket) in TGV is no longer possible and that you have to buy the hole ticket. But this can’t be true theoretically because you can book just the seat reservation on the interrial website.

That is 100% sure. DB say reservation is mandatory only from Saarbrücken:

In addition, it seems the final timetable is now known (it was probably in today's update; there are 3 updates a week, one of which is early on Friday). TGV 9552 will not run between Frankfurt and Mannheim.

The woman you spoke is wrong. DB cannot book Interrail reservations for domestic trains in France, but they CAN book Interrail reservations for international TGVs between France and Germany. You could suggest (in a friendly way of course) they check their documentation...

If the ticket office remains incompetent, then you can try this travel agent. They're quite good, don't have extra fees, can book the same things DB can book and also offer Bahn-Tix (collect at a DB ticket machine).

Thank you very much for your answer ! I’m really happy that this community is very friendly and competent. I’ll try booking it this way.