You have to book via Interrail Website and not via the app, and when it doesn’t work there, you can book directly via sj.se, but you need a paper pass cover number, you get this via DM on Interrail.eu social media. 

Psst: as for now SJ accepts any invented nr=I+ 8 digits. But if we repeat this too often someone in STO might wake up and ask the IT to clse this loophole.

Its also 2 trains+pay 2 times. Perhaps 1 of he 2 is Non SJ? double check!