Root GEM-702L using custom recovery method (TWRP) + SuperSU

Hi
My X2 GEM-702L bootloader us unlocked. I verified by "fastboot oem get-bootinfo" already.

I prefer to root it using the customer recovery method as I'd prior experience using this method on other Android smartphone.

However at the point of flashing the custom recovery I encountered an error:
C:> fastboot oem get-bootinfo
(bootloader) unlocked

C:> fastboot flash recovery twrp_recovery.img
sending recovery (2672 KB)......
OKAY [ 0.609s]
writing recovery
FAILED (remote: Command not allowed)

Where did I go wrong ?