Online, you can book on eurail.com (€2 booking fee per person and train) or via b-europe (€4 booking fee per order):

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

You can also do the entire journey reservation-free:

I have tried to ring the spain number to see if it is possible to make a seat reservation to Barcelona using a visa card but I don't speak Spanish and I couldn't understand the message. I really need to get from France to Barcelona on 17 May somehow if anyone can please give me some advice if it's possible with my eurail global pass, thank you.

17th May is the start of a holiday weekend so a lot of people will be travelling. Book in advance.

There is the reservation-free itinerary as suggested by rvdborgt above. You simply add the journey to the app, board the train and find free seats. :)

If you want a faster journey :

• IC Marseille St-Charles - Montpellier St-Roch 11:24 - 13:04 10€ seat reservation (+2€ booking fee)
• TGV Montpellier St-Roch - Barcelona Sants 13:20 - 16:34 25€ seat reservation (+2€)

All these reservations can be done on eurail.com (2€ booking fee per person per journey. Marseille - Montpellier and all other domestic TGV/IC can be booked on b-europe.com as suggested too (4€ booking fee per order).

Panic not - your pass is valid on almost all trains in Europe,  and many of them do not require reservations.

For domestic TGVs in France there is a dedicated B-Europe site that allows you to book them online and pay with any bank card. They are issued electronically.
 

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

There are no direct trains to Barcelona from Marseilles and the one high speed connection from Paris has a very high reservation fee and only goes twice a day. So you are advised as in previous post to use the Regional train routing without the need to have a reservation.

As in the previous post you can simply add the regional trains to your trip and transfer them to your pass to travel on them without reservation.

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Thank you so much! I have added these res free Journeys to my trip. If I activate my pass now and toggle these 3 trips on will I be guaranteed a seat or do I still miss out if the train is full? Sorry I'm still trying yo get my head around how it's going to work.

You can make all reservations online via eurail.com (+€2 booking fee per person and train).

Calling Spanish railways won't help because they don't sell reservations for this train anymore.

Other options for booking a reservation from France to Barcelona are listed here:

To book Marseille to Nîmes online, see my previous comment. To book both trains by phone, you can call any of the railways mentioned in the link above. SNCF is a bit picky about which credit cards they accept from which country; the others aren't.

I'm guessing you're looking at the journey with departure from Marseille at 11:24. That will cost €10 + €25 per person in reservation fees and €4 per person in booking fees if booked via Eurail, so that's why I also posted a reservation-free alternative.

Nobody will have reservations so simply board the train and find free seats. I don’t think these trains will be very crowded and you’re boarding at the first station the first and last train anyway. Don’t worry.

If in the train there are no seats left ypu have to stand. Be in advance at the station to be sure to get on the train. It can happen that a train is full, but it quite uncommon than no one gets on it.

On the reservation-free journey I posted no reservations are possible. Toggling a train in the app doesn't book any seat. It's just the digital alternative of writing a train on the pass cover of a paper pass.

The reservation free option sounds awesome but I don't understand if it guarantees me travel or if I should reserve if I have to travel from Marseille to Barcelona on that day? Will the res free only work if each train has empty seats?

Thank you all I was panicking but I am feeling much better about the pass now really appreciate the advice!