Download the Fastboot ROM .tgz again from a reliable mirror.
This error occurs when Xiaomi Mi Flash (or a related fastboot flashing process) cannot locate the script file named flash_all_lock.bat (or flash_all_lock) required to relock the bootloader or flash a fastboot ROM. Causes include wrong ROM folder structure, incorrect ROM package type, incomplete/unzipped ROM, misnamed files, or using the wrong flashing tool/version. mi flash error cannot find file flashalllockbat