wich train do you have to take?

If it is a SJ Highspeed train you can reserve directly via SJ on their website using Interrail as discount card. sj.se

@AnnaB maybe you can help more

Boras-Kalmar at 1709 (RE339). When I use the sj website it says ‘ something went wrong’.  I cannot find any way to just make a seat reservation.  Thanks 

Yes I tried it, and got the same error.

But because it is a SJ Regional service should be a error of the app with the compulsory reservation. 

SJ writes it on their website, reservation not needed for SJ Regional:

https://www.sj.se/en/we-offer/interrail-and-eurail-pass.html

Thank you so much.  That’s brilliant 

In Sweden you need reservations for night trains, high speed trains and IC trains.

The app contains errors and says that some regional trains require reservation although it often not even is possible to make a reservation for those trains.