Ensure MAME is looking in the correct folder. You can verify this in your settings or by checking if other games in that same directory work. Use Parent ROMs: While some users suggest you need both

You can resolve this error by ensuring the required file is present in your ROM directory.

If the library was working previously and suddenly stopped, the database likely lost track of the file.

If you downloaded the library recently, the download might have been interrupted. Large sample libraries are often delivered in compressed containers (like .rar or .zip ). If the extraction process failed, or if the installer crashed halfway through, the 463 mk3 ultimate.u64 file might simply not exist on your hard drive yet.