FBNeo is very picky. Unlike some emulators that just look for a file name, FBNeo checks the of every file inside your ZIP. if a single byte is different from what the developers documented, the emulator flags it as "unknown" to prevent glitches or crashes. How to Fix the Error (The "Free" Way) Identify the Required Romset Version
: FBNeo is updated frequently. If your romset is old and your core is new (or vice versa), the naming conventions might have changed. Using a tool like ClrMamePro romset+is+unknown+fbneo+fixed+free
To resolve this issue, you can try the following solutions: FBNeo is very picky