As the seats on this TGV are limited for Railpass holders you get the error maybe because that all seats for Eurail users are soldout You can go with regionaltrains via Portbou/Cerbere to Perpignan/Montpellier and take from there a domestic TGV within France :)

If I recall correctly, there's an issue with booking this train (9:10 from Barcelona) via the Eurail reservation service.

@Nanja, is there a planned fix date yet?

Apparently, if there is another way making a stop in Valencia.  Now I will investigate more in case it is not possible to specify my first option.  Thanks!

Hi Rvdborgt, it has been checked but no issues were identified, meaning that if this message appears the seats are sold our for Pass holders. 

Hey,

I just tried on the interrail website to order a ticket for this train in 2nd class, and I succeed. I get to the page where I have to press the pay button, so try it again!

I just experienced a period of about 2 weeks where there was no way out of Spain into France through Barcelona - part of it was track closures - which compounded the problem of all the pass-compatible seats being scooped up. 

There was track working being done between Perpignan and Montclair that cancelled all trains for a few days.- but the closures on different parts of the tracks were not on the same days (e.g. 3 days for one section then 3 days for the next section with some overlap, etc.) None of this was listed on the Eurail Planning app nor Oui.SCNF. There were announcements in French on a train 12 hours before that essentially it was the last train out, and some poster in the station in French. 

So you might see if you can take a train to some mid-city along the route then bus around a track closure if thats the problem. The bus is actually quite cheap, but the bus terminus is not always the same as the train station. And we got hit by a tram strike the one day we needed to do this