I had the same issue too. I tested the app on AppSupport before buying a ticket and all looked fine. I bought an interrail mobile ticket via the website afterwards, having full faith in the Sailfish AppSupport, but I could not add it due the lock screen requirement. That came as an unwanted surprise to me.
I had to take an old (ungoogled/LineageOS) Android phone with me on the travels, with a broken screen held in place by tape, just for the mobile interrail ticket. But I ended up having a nice trip. Although next time I am going for the physical ticket that they will mail to you.