Suggested fix (unconfirmed): Replay the Cathy Vol 1 levels and get a Red Ribbon by beating the level without missing a note and hitting every note as Great or Perfect.
No confirmed fix yet. Players suggest: it looks like they did a patch which fixed some mods so they work now others might be broken still the few i checked were fine and i got several running which earlier wouldn't download so give it a shot?
Suggested fix (unconfirmed): Verify the game files through Steam and restart your Steam Deck in desktop mode. Ensure your mod manager and game are both installed on the same internal drive.