Summary
Ensure your console has at least 50% battery life or is plugged into a charger while running the tool to avoid a mid-process power failure.
If you're using CFW, you might need to reinstall it. Make sure to follow a reputable guide for your specific CFW version, as the process can vary.
When you download a standard Nintendo 3DS game ROM, it only contains the game data itself. It does not contain the internal operating system files of the physical Nintendo 3DS console.
The game fails to load the specific Mii face textures/models.