Have you read the guide on how the mobile pass works?

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

The reservation you make on the website will not turn up in the app as the reservations are separate from the pass in the app. 

The pass in the app is used to create your ticket.

If you have more questions after having read the instructions, please ask here again.

Sorry Anna, i am not understanding how is the relation of the app and the website works. Can you explain to me, please?

And my pass is not the Interrail Mobile Pass, its Eurail Global Pass...

The Interrail and Eurail passes work in the same way, but here is the guide for Eurail. 

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

Please look at the guide first and then get back with questions. 

Okay, didn’t answered my question, if anyone else knows the answer please, help me

Nothing. If it's a mandatory reservation, just have it ready to show during ticket inspection. Reservations do not appear in the app.

You board the train and you find a free seat. In 99% of cases, that works fine.

99% of the trains can be boarded without any barriers or checks. For the other 1%, there are sometimes gates, which you can either open with the QR code of the app, or that will have staff near them, whom you can show your pass (the QR code or the readable information below it).

Thank you rvdborgt I’ll just have to trust the process.