1. DB can book it 100% sure (ticket office or by phone; in the latter case they send it by post or you collect it at DB ticket machine). What did they say the problem was?
2. What did NMBS say? They claim they can book this train, so I'm curious what the problem was.
3. SNCF will only sell it at the station because since August 2021, they refuse to sell interrail reservations by phone for reasons unknown (although they can technically still do it).
4. RENFE can only do a pre-reeservation and then require you to pick it up at a Spanish station so that's useless when going to Spain.

Maybe. Probably depends on if the ticket office wants to see the Interrail.

Final option: book via Interrail and also pay 2 EUR booking fee + 9 EUR postage fee.

By the way, the price of this reservation depends on the distance. So if you have the choice of where to change into this train, then change as late as possible.

For which exact train (departure time) is it? The one leaving 14:35? 15 April is the Friday before Easter so it will be quite busy.

To ask some one In FR: can be done very easy for INland trips for TGV (On the machines), but this is NOT possible for INTernat. trips=you have to visit counter and show pass.

@Elsemiek Yesterday I was seeing no availability for any train to Barcelona on any day. I figured it could be due to the problem with the Interrail reservation tool but it could also have been a problem with the French or Spanish reservation system, and maybe that's why nobody could book. Today, I'm seeing available seats again, e.g.:

So I'd suggest to try again via DB or NMBS. Do let us know if it worked.

@rvdborgt: Thank you for your quick replies! And you were right I think! Suddenly I could make the reservations yesterday evening via Interrail when trying again. The paper reservations are on our way now:)