Don’t panic.

When you use a pass there are some trains with a mandatory reservation needed, which is extra to your pass. You have to show both reservation and pass to travel.

Some have optional reservations, where you can book one if you wish.

But many do not require a reservation, and in fact do not offer one.

When you use the planner or reservation tool, either on the app or the website it will advise you which category it fits into  and if it states reservations not possible you simply add the journey to the app and turn up and go.

Compulsory reservations tend to be the high speed, long distance and cross border trains.

If in doubt post the details of the train you are interested in and one of us will be able to advise you.

I just had the issue right now lol! 
 

I went in a train thinking I didn’t need anything but the ticket inspector asked me to show a QR Code (that I had booked the itinerary!)

Like Yorkie said above, you need to add in your pass the journey AND activating it on the mobile pass which will generate a QR Code

refer to this tutorial made by Interrail, the « add your journey to your pass » part : 

https://www.eurail.com/en/eurail-passes/eurail-mobile-pass/getting-started

@Cecilia Beccarelli

If you give the travel details  (route, departure time and date) you will get advice regarding the reservation.