I can’t understand what to fix the problem

This is what I am told
Required tools:
A 3DS with Homebrew Launcher access
The 3dsutils homebrew app (Click here for a non-official version)^
An SD card reader or a way to wirelessly transfer files between the 3DS console and a computer
^ There is no official precompiled version of this app, but a non-official build is provided for convenience. Alternatively, you can install devkitPro and compile it yourself from the 3dsutils source code.

Instructions:
Run 3dsutils via the homebrew launcher.
Press A when prompted to begin. The system archives will be dumped. Wait for the process to finish, then press A when prompted to be taken back to the homebrew launcher.
There will now be a 3dsutils folder at the root of the 3DS’s SD card. Inside that folder is a folder nand. Transfer the folder nand to the User Directory. The folder will merge with the existing nand folder contained therein. If prompted to overwrite any files, say yes.

My question is I use Mac so I can’t understand how to do with it and I can’t find any video to teach me on youtube

Do you have a 3DS? If you do, do you have Homebrew in there?

I now know how to fix it but I can’t find where to put the folder

If you have dumped the files from the 3DS, then you’ll find a folder “3dsutils” at the root of the 3DS. Then, inside it is a folder nand. You said you have a Mac, so you’ll find the user directory on ~/.local/share/citra-emu/. Replace the nand find with the one from the 3DS.