fastboot flash recovery twrp-3.x.x-x-<device>.img fastboot reboot recovery
If you are seeing the "Root not available" error, the following steps are the standard remedy to install SuperSU and perform the root operation properly. root not available install supersu and perform root first
Without custom recovery, you cannot install SuperSU properly. fastboot flash recovery twrp-3
It means the app is checking for a working su binary and a management app. You must complete the rooting process (unlock bootloader → flash recovery → flash SuperSU/Magisk) before that app will function. fastboot flash recovery twrp-3.x.x-x-<