Your English is okay. I understand you my friend. Okay. The command you are actually supposed to execute is:
fastboot flash boot magisk_patched.img
I know, it is a bit confusing, because Magisk isn't actually being installed to /boot per se, but instead it's going to /recovery. But, Magisk patched the boot image so that it will install correctly when you chose "Recovery Mode" during the patching process. Flash it to /boot instead of /recovery and you should be good.
The bad key notification is normal. It isn't an error. It just means your bootloader is unlocked.
Once you flash the patched boot image, you will need to boot your device as if you were booting into recovery mode. Again, I know this is very unorthodox compared to the traditional way of rooting, but booting your device into recovery each time you need to reboot will boot your Android OS with Magisk installed and active.
@Spaceminer, if I'm missing anything please correct me, or any other members with any input on this. I'm going strictly off memory on these steps. I know you must use fastboot flash boot magisk_patched.img, but I hope I'm not missing anything after that.