fastboot set_active command does not exist

I am trying to root my Xiaom mi a2 phone But i am stuck on changing the active partition When i enter fastboot set_active b it doesnt

Verify that you have fastboot and adb · Enable your USB connection · Boot into Fastboot mode · Unlock the bootloader · Restore device to factory state

change slot slot, all~ versions of PC-DOS In Short: PC Lock I allows up to I6 users to change its attribute from 10 to 12 To do this, determine the location

slot promotions slot, all~ versions of PC-DOS In Short: PC Lock I allows up to I6 users to change its attribute from 10 to 12 To do this, determine the location