Are you seeing this error on a specific device like a or Steam Deck , or are you using a PC front-end like LaunchBox ? FBNeo not working with "Romset is unknown" : r/RetroArch
Some older games (like Donkey Kong or Galaga ) use "Samples" for sound. If the emulator sees the ROM but doesn't see the matching sound samples, it may flag the set as incomplete or unknown. The Arcade vs. Console Rule fbneo romset unknown