The “good content” = , not a random MAME or old FBA set. Use a ROM manager; manual fixing is painful.
Many "unknown" errors occur because the emulator can't find the system's BIOS. For example, you can't run Metal Slug without the neogeo.zip BIOS file in the same folder as your game. romset is unknown fbneo fixed
The following issues are the most frequent reasons for this error: The “good content” = , not a random MAME or old FBA set