Did you pasted in the "game" folder? I mean
SteamLibrary\steamapps\common\First Snow\game <---This folder

I'm having the same issue. When I first played back in 2020, the 18+ patch worked fine. However, now that I come back to it, the rpa file is in "game" but the game boots up the vanilla version of the game.

So I somehow figured it out. You have to right click on it and choose "Browse Local Files." In my case, it wasn't there even though I had downloaded through my files folder on my desktop. I then redownloaded it from their site, kept the "browse local files" tab open, and moved the dlc.rpa file there. Then it worked for me.