Yes RENFE ticket counters cannot issue them anymore since SNCF ended their cooperation.
Usually you should be able to book them on interrail.eu (with 2€ booking fee per person) but due to a bug it doesn't work at the moment. Depending on your travel date either you wait a few days or call SNCF : https://www.sncf.com/en/customer-service/contact-us/telephone
The TGV between Spain and France is incredibly popular and expensive as there are not many trains per day… it sells out well in advance.
When would you like to travel ? As long as you can buy regular tickets on sncf-connect, you are able to obtain seat reservations (25€ to south of France, 35€ to Paris).
2 alternatives if the TGVs is full :
- regional trains Girona - Cerbère - Montpellier/Nîmes + TGVs to Paris (10-20€)
- AVE Girona - Lyon (runs Friday-Monday until September)* + TGV to Paris
Btw reservations can never be made on the app, you're redirected elsewhere.
*this train should be available at a RENFE ticket counter for 10€. Do let us know if you go there, feedback much appreciated