No reservation needed. Do you have a screenshot where it says fully booked?
Used this train a couple of times in the other direction and never needed a reservation.

I was checking it on SNCF, I'll try to get a screenshot, but the website is not loading at the moment 

Until 1st August I also don’t see any booking options on sncfconnect.
But when I check one date where it is possible to book, I get this “Free seating”:

Maybe the trains are cancelled and that’s why it is not possible to book?
I read some time ago about problems with locomotive drivers because of language requirements on this line… not sure if that’s already solved. Maybe someone else has more info on this.
But if the trains are running then you can go without reservation.

Thanks for the reply. I'm hoping there's at least a train in the morning so we can go to Barcelona, there're a few that also appeared cancelled. Is there an alternative route from Perpignan to Barcelona or eventually from Narbonne to Barcelona?

Well, you have the option to go with the yellow train “Train Jaune”, but that’s maybe more time consuming because it doesn’t run so often. 
https://letrainjaune.fr/ 

Never used by myself this one but should be very beautiful and in summer with open carriage, so if you have the chance, go for it .

I was once traveling by train to L’Hospitalet pres l’Andorre and taking bus from there to Andorra, then taking another bus to Barcelona airport and from there by suburban train to the city… but I read somewhere that the bus from L’Hospitalet stopped running. But hitchhiking should be well possible from there to Andorra...

So or so good luck! I am sure there will be some way.

I'll check it! Hitchhiking is not really an option, I'm travelling with two kids, but we'll seek options if need be. Thanks for your help!

Agree with Martin’s idea, if you’ve the time, go over the mountains. The Train Jaune  goes up to Latour de carol from where you can take a direct local train to the center of Barcelona. This last one is not in the Interrail Planner but you can find it in Google Maps and it’s included in your Interrail pass.

You have to know THIS; Coming from FR last stop IN FR is Cerbëre, first stop In ESpana is Port Bou.

So-logically: C->PB is over the border. You can only do that over rails in tunnel-the road is very far away.

In the distant past-also due to the different gauges in FR and ES it went always like this:

You always use train till over border, furthest, as there the customs checks were done and train onward would wait other side of platform-look and see wider rails. FR train would come back empty to C, ES train ditto to PB after setting off pax over border.

This has all changed. But apparently now there is a EU (and probably by the unions-on trains always seizing every chance to protest and get higher wages and more staff working) rule that train driver must be fluent in both languages-and that is a very hard thing to swallow for FRench. Even though over there about anyone speaks local dialect, catalan, which is the same both sides of border.

So trains now stop last station In their own country. There are/were a few ES trains that woud do the run over border, but the exact actual situation is unknown.

Just checked the FR TER-occitanie site (should be evident for anyone to start there) and it clearly announces yet another STRIKE from 23/7 till unknown last date. The FRench in their ´try to be nice to anyone´ special PR-talk call that ´mouvement social´. With very heavy disturbance of all services expected.

I confirm the strikes, our travel plans have been affected last three days. Today we meant to travel between Nimes and Avignon (and onwards) and our train was cancelled, we took another train two hours later as all the ppl, it was packed, old people having to stand, vive la France:-/

@mcadv fake news. The regional trains have continued running across the border. There was never any mention that this would stop. Check before you post, please.

The only train that does not continue to Portbou anymore is the night train from Paris.