Need to reserve a seat on june 2, from Barcelona to Carcassone, SNCF don’t let me do it online, i am in USA, and afraid to get to june 2 and can’t travel.

SNCF doesn't sell pass reservations online anymore.

You can book them via Eurail though. Did you try that?

Your fears are utter nonsense, but tipical for USA-newbees.

I assume you stay the odd day in Barca before? (I did too last week). Then simply go to the big main estacion Sants and show pass and tell @ counter your wishes-thats by far the easiest way. there are several trains/day, some with need to change (Perpignan, Narbonne)

Can also do for ´free´ without any REServacion by using the slower local trains, RE to Cerbëre, change to SNCF-TER, change again Narbonne. saves a bundle too, as RES across borders are quite pricey as supplmt.

I don’t care about paying an extra 48 or 58 euros I have the money BUT i like to pay ahead and have a Confirmation of my seat.

I did try the reservation, BUT is this guaranteed? Will SNCF respect my reservation vía Eurail App? I don’t care about paying the extra fee of 48 euros, I realize afterwards, that Eurail has not a good contract with RENFE, SNCF and trenitalia.

Yes, a reservation guarantees your seat in a specific train. There are a number of reservation systems in Europe and many different sales systems connect with them to book reservations. This is necessary to make international travel work. Eurail also connects to various reservation systems, e.g. SNCF's.