Dzien dobry.
There is an extensive overveiw by seewulf on how to do REServ outof the app-which you shouldalways only use as very last resort.

For ESpana: only locally at counter

Fir IT; use a sideway via oebb.at-select ticket-then add pass.

Fr TGV-sometimes, if lucky via app, if not, call SNCF, or if it is IN FR and for a few days later when IN_at machine.

You've chosen the least Interrail-friendly countries: Spain, France and Italy.

French TGVs (als from/to France) are best booked via SNCF by phone: Press #85 for English, no booking fee, reservations are sent via e-mail.

Spanish domestic trains cannot be booked online; here is an overview of the booking possibilities.

Italian night trains should be bookable via interrail (+2€ booking fee) but the booking website is having problems again. You can also book via Belgian railways by phone. They will send the reservation by e-mail. You can also book via Deutsche Bahn by phone (+49 30 2970) but then you'll have to collect the reservation in Germany from a DB ticket machine or have it sent to you via the post for €5.90. Maybe also the PKP ticket office can book it. Just ask.