This train is bookable via the Eurail website but they can only issue a reservation on paper, which will need to be sent via the post.

You can also book this train via SNCF by phone. Press #85 for English, no booking fees, reservation is sent via e-mail.

It is indeed wise to book this train in advance. There are also reservation-free alternatives via Portbou, by the way.

Thank you! I called the number and the representative was very nice. The only issue is that you can only pay with an American Express credit card, which I don’t have. I will just have to book this ticket regular price.

I think you  can book at travel.b-europe.com for trains run by SNCF.

here, you need pass cover number which is only on paper eurail pass. 

if you have mobile pass, you need to ask customer service to provide you one which will take some time..(at which phase I am currenly stuck at)

@zarawatkins @darknght92 

You can get your Pass Cover Number from Customer Support by using this form:

https://eurail.zendesk.com/hc/en-001/requests/new

Customer Support is currently overloaded with requests so you will probably have to be patient to get help.

When you contact Customer Support, please mention the date when you plan to start travelling so that they can prioritise your request correctly.

This seems strange. You must be able to use Visa or Mastercard as well as those are the most common credit cards in Europe. 

As @rvdborgt wrote there are other ways to go from Barcelona to Nimes where you don't need a reservation. Please ask if you want advice on that.

Thank you for your kind concern and comments.  Actually I did used above form, and also sent private message here too.  It has been only 36 hours, and I understand the customer service must be flooded after long travel restrictions, plus may be staff has been reduced.  I will be patient and wait few more hours, and hope I get response within 2 days as stated  in the automated reply.

@Marion Help with of Pass Cover Number needed.

That is very strange. Did the operator say that or were your Visa and Mastercard declined? In the former case, I'd just call again and retry. They definitely accept other cards. In the latter case, you'll have to contact your credit card provider to ask what is wrong.

Only domestic trains in France plus Thalys/Eurostar.

The rep said they only take American Express for those in the US. 

Still very strange. I'd just call again. Chances are you'll speak to another staff member.

If they say again they only accept AE, challenge them: why is that? Don't they want customers then? How are you supposed to book, since by the time you get to France (or Spain), reservations will very probably be sold out.

I find it very hard to believe it is blocked technically.

Maybe try your best British accent...