Yes to reserve this route is difficult, via eurail.com they send you the reservation via postal mail. If you call the call center of SNCF you get it as e-reservation. 

Additionaly, there are only a quota o seats im this trains for Pass users, and they are often sold out, because there is only 1 TGV Barcelona-Paris per day.

Here some further informations about where you can book reservations for this route.

Thanks so much for this information...

So what do you do if you can’t get a seat reservation?

You can use local trains Barcelona-Cerbere and change on TER Cerbere-Perpignan and from there TGVs to Paris.

The local trains till perpignan do not need a reservation and TGV Trains inside france have not quotas for pass users and you can reserve a seat if the train is not full booked (but all seats are available). And they are online available via eurail.com or others like travel.b-europe