For the most up-to-date and specific instructions for your console version, always refer to the official 3DS Hacks Guide Troubleshooting Are you currently using the method or are you from an older mod?

: On some operating systems, users might accidentally rename the file to SafeB9SInstaller.bin.bin

Windows, Mac, and Linux handle file names differently. Often, people download the file and it gets automatically renamed with an extra extension.