This train is annoying. You can look through the forum for the dozens of threads about it.

Basically reservations are only available at Spanish ticket counters* or on the day on the platform by speaking to staff.

Availability is usually fine: staff allows you to stand at the bar while they look for free seats. Arrive in Lyon at around 1:30pm and go straight to the Barcelona train coming at the same time.

On a summer weekend I would consider that plan B: buy TGV reservations in advance and cancel them if you manage to get reservations for the direct train (fully cancellable until 30 min after departure).

- TGV Lyon Part-Dieu - Valence TGV 14:10 - 14:45 (unfortunately no later departure...)

- TGV Valence TGV - Barcelona-Sants 16:57 - 21:27

*a friend living there could buy them for you.

Hi thanks for this. It is really frustrating!

So could anyone living in France or Spain buy the seat reservations in advance? 

Many thanks

At Spanish ticket counters yes. They'll need your name, date of birth and ID/passport number (and perhaps pass number).

But not in France, even though the train starts in France? So confusing!!

Yes not in France. RENFE (Spanish state operator) and SNCF (French equivalent) don't cooperate anymore.

SNCF has ticket counters in France while RENFE in Spain.

Now if only RENFE could make that train available on the Interrail/Eurail websites...

Thanks for asking the question, “cjones” and for your active answering over the past couple of years of comments, “thibcabe”. Both the inability to buy the reservation for a Spain-bound train outside Spain () and the lack of official online clarity are ridiculous. At least, the Eurail folks should have something about this in their FAQ! 

Yes I appreciate the replies. Thank you! but am still so confused about the inability to book this reservation!! What I’ve decided to do is change my trip slightly and go to Lyon the night before giving myself more time (previously our connection was pretty tight and without the reservation I was panicking) Don’t want to risk travelling to Barcelona in the height of summer and missing the connection or not having the reservation. As an extra back up I’ve bought separate tickets with reservations  obviously at an extra cost I can do without!! If we do manage to get the reservation at Lyon I will be able to get a 70% refund. 
Appreciate the advice and will plan differently next time!!! 

You can get 100% refund on French Interrail/Eurail reservations.

The Spanish train and staff only arrive at 1:30pm. So it's totally fine to arrive with the 1:30pm TER from Geneva.

Good luck! I've only read positive reports.

Btw, I also called the Renfe “Help” line (+34 91 232 03 20 - follow the prompts for English, then press ‘0’ to skip the lengthy information sharing and privacy disclaimer). I said I have a Eurail pass and asked how to book the mandatory reservation for the AVI 9742 from Lyon Part Dieu to Barcelona-Sants. She repeatedly confirmed that the only way to do it is with the train staff at the platform or on the train. She said it’s “normally” not a problem. There is no fine if you do this (a concern mentioned in some other threads). Also, I’m travelling in October, so off-peak - she agreed it would be even less likely to be a problem then than during the summer peak.