You can't make any reservation in the app. The app with the pass is your ticket.  Reservations are separate from the ticket and they can be made in several ways.

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 post, and you will get advice. 

Thank you so much for getting back to me. I’m gonna try that website and see how that works.

It is not a website, but a post in this community with information about other ways to make reservations.