It is required to have a reservation and with interrail also have paid the substantial fee for these trains.

If you get on without a seat reservation and the very substantial supplement you could be in  big  trouble.  You’ll be charged the full  fare for the journey plus a penalty for travelling without a ticket.  There are other routes. You can go via Toulouse and over the mountains to La Tour de Carol and down to Barcelona. Or a domestic TGV to Perpignan and local trains to Port Bou. The TGVs need advance reservations but at a maximum of  20 euros if  all the pass holder ones are gone otherwise it’s just 10 euros. Good luck 

Info above was for travel from Paris. There are local TER trains to Port Bou where you change to a local train into Barcelona. No reservations required