If you are looking for a document related to a specific software fix or a purchase receipt, you may want to check your email history or the "Help" section of the specific platform where you encountered the code.
If you want, I can:
Search for "errfix.3dsx" or "MiiFix.3ds" on community forums like the Tomodachi Life Reddit . It is often shared as a direct download link by community members. errfix3dsx exclusive