I recall having issues like that when I tried to do things the wrong way. The forum posts just tell you wow to do something, but they rarely question if it’s the sensible thing to do in the first place. If Standard Notes isn’t in my repositories or if I can’t find a flatpack for it, I would just ditch the whole idea and switch to whatever note app is easily available. If something is not easily available, I just ignore those apps completely. Going through hours of trickery and hackery to build a wobbly tower that will collapse next week is not worth it.
Oh, there’s a flatpak for it. That would have solved the problem very easily. However, the thing is, that many new users try to do things the hard way, or they end up trying to something it isn’t even worth doing.