For emulation to work, your ROM files must match the version of MAME you are using. A is a collection of games dumped for a specific MAME version. Metal Slug 6 (filename: mslug6.zip ) was included in the 0.139u1 romset. If you attempt to use a ROM from a much newer or older version of MAME, you will likely encounter missing files or error messages.

To play this game on an emulator using version 0.139u1, you must ensure your ROM is specifically from the MAME 2010 (0.139u1) reference set . Roms from newer or older versions of MAME often fail to boot due to different file headers or compression methods.

You likely downloaded a ROM meant for a newer version of MAME (like v0.250+). You must specifically search for "MAME 0.139u1 romset" sources.