As you go by ferry from Stockholm to Helsinki, you don't add the travel to the app as you don't use a travel day. You only get a discount on the ferry so sometimes ordinary tickets bought well in advance are cheaper as the discount is on full price tickets. 

Thank you so much for your response. So, may I need buy a ferry ticket from Copenhagen to Oslo too, am I right (if i do not want to use a travel day)?

I did not know that a ferry trip is also count as a travel day. 

You will need to buy a ticket for the ferry. You use the pass to get a discount on the full price. Sometimes it’s easier or cheaper to buy a ticket without the discount!

You don’t use a travel day.

Any travel for which you only get a discount doesn't require a travel day. A travel day is only required when the travel is 100% covered.

Thank you for your help!

From Copenhagen to Oslo I guess that you will go by train? And then you will use a travel day. 

Yes, I'll go with the train, but I thought I can go with a ferry too. As I mentioned, I did not know that using ferry counts also as a travel day.

Do you have any recommendations for a scandinavian tour like which route is the best one or which journey would make possible to see the northern lights?

No you've misunderstood. Taking a ferry does not require using a pass day, even if you get a discount.

The Stockholm - Narvik night train is the best way to go up north. Book in advance through sj.se (add Interrail as a discount).