Disable Drivers Signature Verification in 64 bit PC, Now open Mi Unlock tool and Click right Upper Corner Setting button, and Install All Drivers, Then Reboot your PC. you need Login your Device and Keep Some days Sync (Must Mi Cloud on).
Now boot Device in fastboot mode and Connect to pc by Data Cable,Now Open Driver Manager in PC to Check does your device showing in list as *Android Bootloader Interface*, if showing like that then you can able to Unlock bootloader.
Just Follow This Thread https://c.mi.com/thread-1635834-1-1.html Note:- After unlock All Data will be Erased. Thanks