You have to use other sites.

1 INn->STO is the Snälltaget, a swedish company that has sprung in the renewed hype for overnight trains. However-from what Ive read else-you canNOT reserve with a pass that way-only from Berlin or HH=Hamburg onward. This sector south is occasional and more like a tourist charter. SEAT REServ cost 199SEK=around 20€ with pass.

2.The overnight north from STo is now run by VY.se (=in fact the ex-Norwegian state rail who won the contract) and you can still reserve via sj.se- DO read first all instructions on how to-and you also can do only seats or couchette=liege, not the fancy 1st cl sleepers.

You probably have to make more: its always best to use the site of the railway concerned-but not all allow direct seatONLY REServations for passholders. About any of the most wanted routes have been covered in thos forum many, many times before.

To book via the Interrail app you go to → more → seat reservation. You will be directed to the carriers website.

You can book the train from Hamburg to Stockholm at

 https://www.snalltaget.se/en

In order to only pay the reservation fee you need to enter the code INTERRAIL in the field "Promotion code"

A seat from Hamburg to Stockholm is 199 SEK and a couchette is 399 or 499 SEK depending on what day you travel.

I'll look at the trip to Narvik tomorrow. 

Hi

You can book the trip from Stockholm to Narvik at sj.se if you have your pass cover number. A seat will cost you 86 SEK and a place in the couchette will cost 265 SEK.

Let me know if you need more help.