Go to the official FBNeo GitHub or website and download the .dat file for the latest release. This file is the "blueprint" that tells the manager exactly what each ROM should look like.
If you have a collection of ROMs that aren't working, you don't necessarily need to download everything again. You can "fix" them using a tool called . romset+is+unknown+fbneo+fixed+free
Unlike console emulators (like those for the NES or Genesis) which are often "forgiving" with file names and headers, arcade emulators are incredibly strict. Go to the official FBNeo GitHub or website and download the