Rooting dell venue 8

Hi , i am unable to root my dell venue 8 3840 ...
I am not able to unlock the bootloader nor install the supersu app in fastboot
when i am trying to unlock the bootloader i use this command to unlock my bootloader
fastboot oem unlock
but after entering the command it is showing this
FAILED (remote: unknown OEM command)
finished. total time: 0.303s
And when i am trying to flash the supersu app it is showing updating system then its showing error with a android guy lying down

What shall i do am i doing this in the wrong way or anything else please tell me a way to root my dell venue 8 3840 , i am a newbie in rooting plz help me tell me a way to root my dell venue 8 3840
i also tried to use tethered cwm then also it is not rooting plz tell me a way and sorry for being a noob