Printing will be fine.

1. What happens when you try to load the ticket into the app? If there's an error message, then please post a screenshot.
2. Where did you buy your e-ricket? In the NS app, on the NS website (ns.nl), or someplace else?
3. Did you use the app/website in Dutch or in English?
4. Did you try to load the ticket right from the screen after you paid? If so, then what happens if you tap on the link in the email? If you do that on your phone, then normally the NS app should open and load the ticket.

Thanks for your help. The problem seems to be that I was working on a computer and the link in the order confirmation email was opening on the computer. I got through to someone in NS customer services and they suggested opening the link on the phone (Safari on iPhone) and when I did that (and did a few more clicks) the tickets loaded into the app.