This is the most common cause. When you click "File" -> "Open" in an emulator like Ryujinx, you are telling the emulator to load a full, standalone game. However, if you downloaded a small file (e.g., 500MB) that is actually just an or DLC , it does not contain the full Program NCA required to launch a game. It only contains the data to modify an existing game.
This is the most common fix for users launching games via front-ends like EmuDeck or Steam ROM Manager. The Problem: the nsp file is missing a programtype nca fixed
Update your Atmosphere and Sigpatches to match your current firmware. This is the most common cause