What date and time did you try to make a reservation for?

Where did you try to make the reservation?

These are the two trains which I will take. I have an eu interrail pass, so I don’t have to buy tickets anymore. However, I would like a seat but I can’t buy this in the app.

You can't make a reservation for a Spanish train online. 

Here us a guide to different ways to make reservations for Spanish trains:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105/index2.html?postid=18419#post18419

OR-be smarter,save the extra-no worries too: use the NORmal trains=Catalunya Express from the main estacion-IN town and not miles from) Town-nO res possible and runs ev hour. Has more stops in Bcn too-maybe even closer to where you want to be.

Note that a single ticket for this local train-and not onward-will cost much, much less as a passday