If you have Luma3DS custom firmware installed, download the latest GodMode9.zip . Extract the luma folder and gm9 folder to the root of your SD card. Also copy the GodMode9.firm to /luma/payloads/ and rename it to x_GodMode9.firm (the x makes it boot by holding Start).
You can only restore a NAND backup that came from your specific console. Restoring a foreign NAND (one you downloaded online) will change your encryption seed and permanently brick your 3DS. 3ds nand download install
A is a special, pre-downloaded image of the official Nintendo 3DS operating system for your specific region (USA, EUR, JPN, KOR). This is the only type of "NAND download" from the internet that is safe to install, because it overwrites the OS but leaves your console-unique keys intact. If you have Luma3DS custom firmware installed, download