You're looking for a complete piece for Metal Slug 7 ROM on MAME Recalbox PC 2021.

Place the mslug7.zip file into the same roms folder.

Because it’s a DS game, you cannot simply drop the ROM into a MAME arcade folder and expect it to work. You need a dedicated DS emulator core (like DeSmuME or MelonDS) running inside Recalbox or as a standalone on PC.

Recommended Emulators for Recalbox (PC) Nintendo DS (MelonDS/DeSmuME). 52.221.178.250