Go online, 'buy' a ticket that's marked for smartcard download (you can buy paper tickets too, obviously) and right at the end when you're about to select where to collect the ticket, you'll be told it has to be three days. The TVM also gives the error if you try and buy a (non season) for today or tomorrow, but then switches to showing paper tickets instead [not that this is always that obvious as I played around with the machine software, before I had to leave because other people had arrived wanting to use the machine].
Videos:
TVM:
https://www.youtube.com/watch?v=Cn-sbcCTHJc
Gateline:
https://www.youtube.com/watch?v=WVUcwBlapz0
What I don't have is the display from the 'wand' the RPI had, which showed her/me my name and the ticket loaded on from Hatfield to Finsbury Park for next week. I will of course find out next week if it works the gates okay.
Edit: Just noticed on video playback that there is a 'I have a Paper Railcard' option that popped up after I tapped my card (bottom right), so maybe I could have selected a discounted ticket. Not the usual place/way for getting railcard fares!
Edit 2: Here's the ticket I bought;
View attachment 23202
I shall have to try later to see what happens when I select that I have a paper railcard..