1. I have never heard of a quota of special passholder reservation for this train, but it’s possible.
2. The issue is probably a special, lower-priced passholder reservation. If these sell out, there may be still be reservations available at the slightly higher regular price.
3. This is a very popular route. Certain accommodations, like reclining seats, may sell out when couchettes or other more expensive accommodations are still available. They probably sell a super cheap on-line special for the combo of ticket and reclining seat reservation. These special offers sell quickly and may mean the reclining seats are sold out.
4. I would buy this reservations soon as you get to Paris. 6 days in advance ought to be OK if there isn’t something special going on in Barcelona. Buying on line from a travel agency will probably double the price of the reservation. So you have some wiggle room. If seats sell out, a couchette will probably still cost you less than buying the reservation on line.
5. Check the French and Spanish national rail company websites. Some poking around and clicking on things like “book only extra charge” or anything that mentions a pass MAY (unlikely, but possible) allow you to buy just the reservation at regular price. It is also possible that there is a special ticket plus reservation package that is only a few euros more than the cost of just a reservation — and cheaper than the cost of using a day of the rail pass. Links to the national rail company websites are in a sticky at the top of this forum and under “transportation” on the “travel tips” tab.