If you're in France, go to your nearest SNCF ticket office. The reservations will also be cheaper, since you don't pay any extra fees.

Otherwise, call German railways (+49 30 2970, available 24/7) or Belgian railways.

Edit: for 1 person in 2nd class, I can get to the payment page for the 10:14 train on 23 December. I can't test 4 persons.

Thank you rvdborgt! I went to the nearest SNCF ticket office and problem solved. They did have 4 seats available and could book them at the ticket office, without a problem. 

Just to let you know that I tried to call both German railways and Belgian railways for 2 days, and could not get a hold of them ever. So, this may not always work. 

@J0rba, just repeating the options for you. Reservations for TGV/AVE France-Spain are only available on paper, not electronically.

AFAIK Belgian railways can send tickets via the post or you can pick them up at certain Belgian stations. Deutsche Bahn can also send them via the post or you can pick them up at a DB ticket office or DB ticket machine.

I am also trying to do this for my family. I arrive in Paris less than 48 hours before planning to go to Barcelona Sants. We really need to book the seats for our family in advance as everything else is arranged(other trains and accommodation as interrail led me to believe this was possible to reserve online), but obviously will not be able get to a station to do this. I have contacted SNCF but they have again reiterated that it needs to be at a station. Really concerned about leaving it late as it specifies there are limited seats available for interrail pass holders. 

I am wondering if someone in France would be able to  reserve the seats on our behalf?

I have tried the call Deutsche Bahn and not able to get through to anyone. 

ok I got through to Deusche Bahn, the helpful person said they are coming up but not allowing to book, so either they are all sold out and they are unable to book that end also?! help?! I feel misled as I looked up all the trains and booked everything else so need to be on those trains. 

On which date?

Tuesday 19th April Paris - Barcelona and return Sat 23rd 

I have tried booking online again this evening in case it is to do with release of seats - and now it states something when I try to proceed with the booking ‘Please try to find an alternative connection. The unavailable leg will be removed from your cart’. Previously it did not say anything just would not let me proceed after I had selected them all and put all my details in. 

There is still availability on 19 and 23 April.

I'd try again with DB. DB can book these trains and people were able to get them from DB in the past (quite recently actually, just 1 or 2 weeks ago).

I'm not sure what DB would mean by "not allowing to book”. Maybe there's an issue with the connection to the French/Spanish system or they're doing it wrong. They should use tariff code 44 (TC44) to book this train.

Hi, yes I saw there was availability on the actual trains, but is it because there are limited seats available for interrailers and those seats lack availability do you think?

If you see availability in the Interrail booking module, then there are still pass holder seats available and DB should also be able to book them. If the pass holder seats are sold out, then the Interrail booking module will show the grey "!" (which can also mean other things but usually means “sold out”).

Hi, what did you end up doing?