to enable the installation and booting of ChromeOS on older computers that use Legacy BIOS/MBR instead of modern UEFI. Core Purpose & Usage Enabling Legacy Boot
tar -xzvf mbr_support.tar.gz
You may have gotten a “fake” or truncated file. Check size – a real one is at least 20–50 KB. Re-download from an official source. mbr support.tar.gz download