Hello,
I also faced that error twice.. Once after I tried to install twrp. And second time when I tried to lock bootloader with same command.
Only thing you can do is testpoint method.
1. Download latest china DEVELOPER rom if you want to unlock bootloader later.
2. Put it in miflash and keep MiFlash opened with rom ready for flashing
3. Turn the phone off
4. Remove back cover (there is a lot of videos on youtube
5. Remove battery jack
6. Touch 2 circle pins with tweezers or something like that (keep tweezers on pins untill you connect it to computer)
7. Plug usb in phone while keeping tweezers on pins, but be carefull not to touch anything else while connecting phone
8. You will hear sound on computer that your usb is plugged, then remove tweezers
9. Refresh MiFlash
10. You will see COM4 or some other number in MiFlash
11. Click on Flash and wait about 250sec untill it's finished
12. After it is finished, connect battery back to motherboard and try to turn on the phone
