I assume it says it is not valid?

You do get 50% just for the standard fare. I assume you did select a special fare (selected by default).

And: You do get a reduction just for the passenger fare (which is the cheapest part), not for accomodation etc.

You do get the same discount as with your raillpass when using the discount code “FLASH”. This one is also valid when selecting the special fare.

This route is very expensive, even with the small passholder discount. It’s mainly aimed for trucks.

I’d suggest taking a Stockholm - Turku/Helsinki ferry (Viking Line and others): those are a lot cheaper.

Yes, sorry I meant not valid - typed too quickly! 

It only comes up with one option which includes the cabin, so I guess the discount won't work anyway? ‍♀️

I think I might have to rethink my trip

The discount is applied to the passenger fare part of the bill. You may check this when trying without the discount. So the discount does work but it doesn’t save that much money.