i follow > this tuto
Don't follow video tutorials, they are often outdated and even incorrect. No safe guide should ever tell you to downgrade sysNAND directly, ESPECIALLY the n3ds. Follow the guide I linked instead, it is much safer and takes all safety precautions to make sure you don't brick your system. n3ds on 2.1 is very unstable and prone to bricking (even to the point where a hardmod won't save you), plus you should never use downgrade files from an untrusted source. Just follow the guide posted above, it is much quicker, much safer and much easier to use as long as you read each step. Since you told me you were on 9.2, you can start here:
https://github.com/Plailect/Guide/wiki/Homebrew-Launcher-(Browser)