Go back to Mi Unlock Status in settings and ensure the account is successfully linked.
Log in using the same Mi Account currently signed into your phone. miflashunlocken7672743
Turn off your phone. Press and hold the Volume Down + Power button simultaneously until the "Fastboot" bunny logo appears. Go back to Mi Unlock Status in settings
Newer versions include the necessary USB drivers to recognize your phone in "Fastboot Mode." miflashunlocken7672743
This is usually a driver issue. Try using a USB 2.0 port instead of USB 3.0, or install the "Adb Setup" drivers manually.