Welcome to the forum.
This infamous message means you're using an outdated Luma release (which is not compatible with 11.8 and 11.9 of the official firmware)
Put the boot.firm file from the latest 9.1
Luma release into the root of your SD.
Next time you have a 3DS question put your thread into the
3DS section of the forum (after using your favorite search engine with such an error message). Short, easy questions go into the
3DS NOOB PARADISE thread.
Assuming you have the latest release of
GodMode9 in the SD:/luma/payloads folder, you should also follow section VI of the
finalizing page of the guide to copy the new Luma to CTRNAND so the 3DS will be able to boot in case of missing or corrupted SD.