Dobry den, Karel.

Dont make your self so much prob: simply go to CD and let them handle it for you. Likely also cheaper and pay in CzK. RREServatioans are completely separate fro app.ticket.

I paid for a reservation through the Polish company Polrail. I did click to "have you Interrail pass?" and I thought everything was OK. But now I have no way to get started in the Inerrail pass when the option to go EN 443 does not appear in the menu

It's no problem, you just enter the journey manually to your trip if you can't find the train.

This train can be booked online via CD. Of course also via a CD ticket office.

As you have the reservation for the train, what you need to do if you can’t add EN443 Prague 22:05 Warszawa Gdańska 08:52 as a journey to your trip in the app is to manually enter a journey from Prague to Warszawa and connect the journey to your pass to then create the QR code that is your ticket.

Here is info from interrail.eu on what to do if you can’t find your train.

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/mobile-pass-faq/what-should-i-do-if-i-cant-find-my-journey-in-the-planner-or-station-board

Here is general info on how the mobile pass works.

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

On what date will you be traveling?

I wouldn't worry too much about the train not being in the app. You only have to add it before boarding the train and by that time there will possibly have been an app update with more recent timetables. And you can always add a train manually if needed.