I can't seem to get the Eurail app to allow me to make reservations on Norwegian Trains that require reservations.
How about booking here - https://www.vy.no/en , the national booking site.
Are you trying to use Eurail because you purchased a Eurail App? Usually people make reservations/buy tickets directly with the railroads, but if this is in conjunction with a pass that changes things.
Perhaps someone on the forum that uses the Eurail App will be able to help you with this.
Here are a couple of you tube videos that claim to be on topic for Eurail Passes
Instead of just saying you "can't seem to get the Eurail app to allow me to make reservations" how about describing what actually does happen/what you see/any error messages and other details... that's much more useful (to someone trying to help you) that just saying you were unsuccessful.
You can also use www.entur.no, the national booking site.