Please do not use the rail planner app to plan. It's often incomplete, out of date and generally unreliable. National planners or apps are much more reliable. For international journeys, bahn.com is a good choice.
The rail planner app is just to register the trains you take. There's no need to do that in advance.
Reservations cannot be made in the app. Look here for where best to make reservations:
https://www.seat61.com/interrail-and-eurail-reservations.htm
If you have any questions about reservations, then please specify route, date and departure time.
As to your screenshot: there is no direct train from Stockholm to Rovaniemi.
There are no trains running between Sweden and Finnland. You can go as far as Haparanda in Sweden by train. Then you need to either walk or take a buss to Tornio in Finnland.
Be aware that Sweden is on CET and Finnland is an hour ahead. More than one traveller has missed their connection due to the time difference.
There is no such thing as a train from Stockholm to Rovaniemi.
While there is nothing to stop you putting a fictional train into the planner manually, it doesn’t mean that train will appear or that you will be allowed on any reservation mandatory train that does run for part of your itinerary. It is not a correct use of the pass and you may well end up dealing with a rail official that does not accept this as valid.
The manual entry option should ONLY be used when the planner refuses to display a train that runs, it should always match the correct itinerary of a train (from local sources or www.bahn.com), it should include the correct train number and it does not exempt you from buying reservations for trains that require them.
For Stockholm to Rovaniemi you will have to take at least 2 Swedish trains Stockholm - Haparanda (reservation required on the first leg to Umea or Boden) then a walk and a bus to Tornio or Kemi in Finland followed by Finnish trains to Rovaniemi.
All these trains (but not buses) are in the planner so no need to add manually. Swedish schedules and seat/sleeper reservations can be made at www.sj.se.
It was an example for a journey. The app also does not show a journey from Stockholm to Helsinki, if you want to travel in March.
I suggest you look at the rail map and search timetables on www.bahn.com to help plan your trip. You could also look at the excellent information on www.seat61.com for help with suggested itineraries.
You can’t expect online journey planners to find the perfect train - particularly on a long journey. It needs a bit of help. You can break the journey down into smaller chunks.
For Stockholm to Helsinki, you’ll either go the long war round by train, as mentioned above, or take the ferry.
