This is possible via raileurope.com (desktop version only, make sure to add your interrail/eurail pass as a discount), but only for the TGV (SNCF) service.

The AVI (RENFE) cannot be reserved online. You have to show up on the platform, and reserve at the staff (if they’re places left).

Alternatively, you can take TERs and regional trains via Perpignan Cerbère and Portbou to Barcelona, without reservation (but takes longer).

There is a direct TGV at 13.17 - 16.31

Reserve seats at www.raileurope.com only on the desktop site. €25 per person.

It is also possible to do that journey reservation free by changing at Cerbere or Portbou.

There's availability on the TGV departing at 13:17, in first class.

The cost is €25 per person, bookable as @BrendanDB says through Rail Europe.

Thanks all