"boot.firm" is out of date
The file boot.firm on your SD card and/or on internal memory is incompatible with the current system firmware.
To fix this issue, just download the latest release of Luma3DS and copy boot.firm and boot.3dsx to the root of the SD card, replacing any existing files.
Additional notes
If it still isn't working, you should check the following things:
- Is the SD card formatted as FAT32? The 3DS can't read any other format.
- The 3DS may have trouble reading an SD card if it was formatted with MiniTool Partition Wizard or the HP formatting tool. Try formatting it with one of the recommended tools (Windows, macOS, Linux).
- Is there dust or moisture in the 3DS SD card slot?
- Do you see a RED or MAGENTA notification LED? These colors indicate file corruption of some sort, meaning that your SD card may be unreliable. In this case, you should check it for errors (Windows, macOS, Linux).