Cannot get to fastboot/bootloader menu on xiaomi mi5
Can't you just use your google account to recover the locked phone?
Anyway, if not, keep the device on, the type in cmd adb devices

If the device is not displayed you should, in effect, flash a ROM, in order to use it again, if the phone asks for permissions give them to it and do adb reboot recovery

Then you can proceed to wipe the data