It's not "our app”. This is a community of people like you, who share their experience.

The rail planner app can't book reservations.

A good overview of where and how to book reservations is on this page:

https://www.seat61.com/interrail-and-eurail-reservations.htm

If that doesn't work for you, then please mention route, date and departure time for each train you'd like to book.

This route is scheduled:

Sept 15 - Paris to Stuttgart - 10:52 - ICE 9573

“seat reservations required” is highlighted and does not accept entry.

What are we supposed to do next?

It’s best not to use the Eurail booking service. They charge an extra €2, and it’s not very user friendly!

You can book this on raileurope.com (desktop only, not mobile). Add the railpass in the search box, and you should see it in the search results for €18.

I think the ICE trains are not on sale yet for September dates, so you’ll have to check later in the year.