Entering the number doesn’t work ?

Yes, please describe what the problem is, and post a screenshot of any error message or of the screen where you get stuck.

Also note that you can book reservations even without a pass and it's better to wait with activation until a few minutes before you board your first train. Nevertheless, you can test activation now and then cancel activation.

I am putting in my passport number - its 8 digits long and it wont let me press next in the app.

Or can I book anyway and activate at station?

Could you please post a screenshot?

You'll need to activate your pass yourself eventually. The ticket office staff can't do that for you.

By any chance, is the problem in step 1, where you enter your last name and pass number?

If so, then please note that you need to enter the pass number, from the confirmation email, not your passport number.