I’m having the same issue when loading .dsk files. The standard green screen comes up and throws an SN error. No .dsk files will load. No issues with .cas files though. Today I installed the emulator from the latest source, dropped the bas13.rom in the BIOS folder, and dropped .dsk files in the coco folder under roms. Retropie version 4.5.1 installed.

@Azurden I believe you are confusing various uses of the word 'set' and incorrectly equating them. You need ROMs that match your emulator's needs. The easiest way to deal with this is to get yourself a complete collection (a rom set) of MAME 0.78 roms. Individual games are comprised of rom sets as well. Different versions of a game will have different roms comprising them. Missile Command set 1 and set 2 are two different hardware versions of Missile Command. Get yourself a full collection mame 0.78, copy the games you want over to the roms/arcade folder on your pi, and then use lr-mame2003 as your emulator on the Pi.