It is not a good idea to use Interrail in Summer in Spain, but also France is not the best place. For Portugal it is not possible to reserve seats online. Only in Portugal. And to get to Portugal is also a nightmare, because there are no direct trains but only with regional trains.
There are some problems with the booking of reservations on interrail.eu
For trains in Spain of RENFE and the TGV/AVE trains Barcelona-Lyon-Paris you can reserve a seat at Ticket Offices of Deutsche Bahn. Same for the ICE/TGV service Germany-Paris.
For TGVs in France you have to book directly via SNCF call center (press #85 for english) or a travel agency in Germany that sells SNCF Tickets. You can also book online via B-Europe but only with paper pass cover number, you can request it also for mobile pass via DM on social media of Interrail.eu. On B-Europe you can also check if there are seats left in the train. But currently (in weekends) almost all trains in France are booked out. Please look if it is possible to not run via Paris but like via Switzerland to Geneve and Lyon and from there it is more easy to get to south France and Barcelona.
The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new topic, and you will get advice.
Here is information about how to get a refund of the pass. This is possible if the conditions for the Pass allows a refund.
https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy
As you know about the slow trains… see it positive and use them - you will save money and have WAY better scenery than from the high-speed trains… and see some beautiful cities along the way.
Way better than by high-speed trains and 1000 times better than by plane! See it as part of your trip
