At the Renfe long distance ticket booths in the main hall.

Note there can be quite a queue (they work with a number system), but it varies depending on the moment. If you're in Barcelona for longer note you can also do so at Barcelona-Estació de França (where queues are probably shorter) and possibly at more stations (I've made bookings in the past at a station that wasn't listed on the list provided by Interrail).

I'm not sure if they can make reservations at the Renfe service desk within the departures area. If you arrive from Paris (and use the stairs instead of the escalators - the escalators lead to outside the security perimeter if I'm not mistaken) that might be worth asking. 

Cannot do the latter-of course can only do when they also accept paying-and that should still include cash. Perhaps can do if foned/RES advance and only have to collect that ticket etc

I also highly doubt if they let you onto an AVE without that airport-imi style luggage check if coming directly from the Francia!?

I wrote the previous comment because I recalled seeing a payment terminal on that desk.

You can doubt it but I did it about a month ago, as long as you have the stairs and not the escalators (the escalators are one way into the publicly accessible hall).