You have to pay the reservation if you want to use this train. Not having a reservation is a bad idea. You could be kicked off the train, made to pay a full fare ticket or pay a hefty surcharge.

There are also reservation-free options via Cerbère / Portbou but these take more time.

Okay thanks, so I‘ve checked, I could drive to Port-Bou for free. But then, no free routes to Barcelona can be seen in the Interrail-App. But on the spanish Renfe-App it shows that there are regional trains driving. But it says I can‘t book them online.

So should I just add the departure and arrival time of the regional train I want manually to my Interrail-App? Is that enough?
Or do I have to pursue the train ticket at the Trainstation in Port-Bou?

Thanks :))

Just add the trains manually the trains from Portbou to Barcelona doesn´t need any reservation :)

Okay alright I will do that thanks.

But why are the regional trains in Spain not shown in the Interrail-App?

Because RENFE is the best railway carrier for Interrailers in the world. 

They do not send the timetable to the European timetable center, where the Railplanner app gets the timetables. 

but atleast Interrailers can still use their regional trains by adding them right?

right

The region publishes a PDF timetable of Cerbère/Portbou - Barcelona here. It seems to be complete: it also includes the MD trains and the SNCF trains that run between Cerbère and Portbou.

All of these trains can be used. For the MD trains, you'll need a reservation (4€), which you can get from the ticket office in Portbou.