Boot into the GW Menu and create an EmuNAND. Be sure to make a backup of the contents of your SD Card, as it will be formatted.
Your other option is using EmuNAND9. Link:
https://github.com/d0k3/EmuNAND9/releases
Boot into it as a luma payload, and once in it, pick: Complete EmuNAND Setup
As always, be sure to make a backup of the contents of your SD Card, as it will be formatted.
Once you make sure it's working, you'll want to install Tiny Format on one and format it, to unlink it from the other.