First of all, the phone must be bootloader unlocked, if you didn't do that please unlock the bootloader first, you can find many guide for that.
And one thing, there is a switch called "OEM Unlock" in developer options menu, make it turn on or you will fail to get unlock token.
Now you have a bootloader unlocked device right?
Please download this TWRP recovery
Let phone reboot to bootloader and switch to fastboot mode then use the command to boot the recovery.
fastboot boot twrp-3.2.1-0-a56dj.img
fastboot flash recovery twrp-3.2.1-0-a56dj.img
I recommend to use Magisk to get root