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.
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.