Not sure if the USB Version of the port in use will impact this, but with other activities using ADB, if a USB 3.0 port is used, there are problems - try with a USB 2.0 port.What ROM version are you using?
Anyway Try this Mi Unlock Tool version: https://drive.google.com/file/d/1OjuuNcL0AtE2-aMZrc0xYQ51cw6WDo1a/view
Also try better quality USB cable and/or different PC
Some days before I unlocked BL on Mi8 for my friend without any issue...
Further info: Enabling OEM unlocking and USB debugging options were not needed.