If it’s one of the trains operated by SNCF (TGV in the search results), you should be able to make reservations through Interrail ledited to add - or, better, Raileurope.com, as @Al_G says]. If it’s operated by RENFE (AVE), unfortunately you can only do it at a station in Spain. It’s very frustrating!

Which train do you want to reserve?

Your alternative, if you have a flexipass, is to use your days for travel somewhere else, and buy a ticket from Barcelona-France.

The direct RENFE AVE train cannot be booked online for passholders, only in Spanish ticket offices.

The 2 daily TGV from Barcelona at 09.28 or 13.09, changing in Valence TGV can be booked via Raileurope.com

Thanks.  Let me add this.  Under My Pass on the Eurail app it shows a QR Code.  At the bottom labeled “Journeys” it does list the trip, but again says seat reservation required.  Does that mean, yes, I have a space on the train but just not an assigned seat?  I guess what I’m angling towards is as long as I have passage, that’s some sort of relief. I could show up at the station a few days prior and get seats.  I could live with that.  
 

…and/or should I follow the advice above and just also book through RailEurope.  That’s $391 for all of us.  If I didn’t need it ultimately I could cancel and forfeit 30% . 

No, that just means that you’ve logged the train with Eurail and you’re intending to take it. These trains have compulsory reservations, and if you don’t have seat reservations, you can’t travel.

If it’s the AVE train, you can show up at any Spanish station (as soon as you arrive in Spain) and buy the reservations there. If it’s a TGV, you can buy them online.

Unfortunately, that’s the way it is with RENFE. They make it very hard to reserve their trains!

Wow!  Your response is amazing.  I think the clouds cleared a little in my mind.  Super asset to the community.  Sincerest thanks.  It’s listed as AVI9737 Barca Sants to Lyon Pt Dieu.  @08:19.

So Im guessing that’s a must book in Spain situation.  And that means although when I walk up (with family) 2-3 days prior to take care of seating business.  I don’t need to sweat getting on or not getting on.  But I could not be together or in the class I want.  Is that correct?  
 

…For that Sunday ride, Olympic and Sunday crowds, would it be an overwhelming best advice to just go ahead and book through RailEurope and  joist pit this topic/worry to bed? …have an unspent day on my Eurail pass at the end?  Again!  I truly appreciate you.  Thanks!

Glad to have helped!

Must it be that exact train, or are other trains an option?

For example, you can reserve this connecting journey via Valence at a cost of £118 ($148) for three people.

If you're willing to take a slightly slower connection from Valence, that cost reduces by half.

Yes!  I will book that.  So basically to confirm again.  When I book this, it doesn’t count as a rail day (Eurail Global) versus the 4 day pass I already have since it’s a direct booking?

Ah, no. That's the price for reservations for 3 people on two trains. You'd need to use a pass day.

Depends whether you get tickets or passholder reservations. The latter are available in many websites and onboard you must show both a valid pass day and the reservations.

The price above would be using a pass day.

No, the price @ralderton linked is the passholder reservation fee, on raileurope only on the desktop site there is the option to add a pass (of whichever class you have).

This is the reservation fee for the 2 TGV connecting at Valence TGV, you can cut that fee by only booking the Barcelona - Valence TGV part and taking non-reservation TER from Valence TGV - Valence Ville  Lyon Part Dieu but this will be slower, arriving at Lyon at 16.40.

Let me also mention that TGV reservations bought on Rail Europe are 100% refundable until 30 min after departure.

That way I'd book those TGV reservations for peace of mind but feel free to go to a counter in Barcelona when you arrive and ask for the direct train there.

(França station is always less busy than Sants btw.)

I think @thibcabe has the winning solution here!

1 Buy the expensive passholder reservations for the 0928 train now.

2 As soon as you get to Spain, visit a ticket counter and buy reservations for the 0819 train. They will be much cheaper - €13 per person. And probably available.

3 If successful, refund the expensive reservations on the 0928, but at least you have them as a backup.