Fastbooot: FAILED (remote: 'unknown command')

Question

I've bee trying to install twrp on my Asus Zenfone Max Pro M1 X00TD. So, first I enabled USB debugging, connected my phone to computer and booted into fastboot by adb reboot bootloader. After that I tried to run command fastboot oem unlock. But the device showed the following error.

FAILED (remote: 'unknown command')
fastboot: error: Command failed

No matter what command I try it responded in the same manner except for the fastboot reboot command which rebooted my device into system. Also in the fastboot mode the screen shows CSC FASTBOOT MODE instead of FASTBOOT MODE. I don't know if this has something to do with the problem.

this is the script I used to unlock my bootloader.

#!/bin/bash

#
#    This file is create by liunianliang
#    For download all img by fastboot
#    2018/01/17
#

# set path
if [ -z "`which fastboot`" ] && [ -e "fastboot" ];then
    for i in `echo $PATH | sed -e 's/:/ /g'`;do
        cp -rf fastboot $i/fastboot > /dev/null
    done
    export PATH=`pwd`:$PATH
fi

# check fastboot tool
if [ -z "`which fastboot`" ];then
    echo -e "Error: No fastboot tool found !!!"
    exit 1
fi

# default platform
default_platform="sdm636"

# default buildtype
default_buildtype="eng"

# default slot
default_slot="a"

# get platform from fastboot command
platform=`fastboot getvar platform 2>&1 | grep platform | awk '{print $NF}'`

# get build-type from fastboot command
buildtype=`fastboot getvar build-type 2>&1 | grep build-type | awk '{print $NF}'`

# get slot from fastboot command
slot=`fastboot getvar current-slot 2>&1 | grep current-slot | awk '{print $NF}'`

secret_key=`fastboot getvar secret-key-opt 2>&1 | grep secret-key-opt | awk '{print $NF}'`
secret_partition=`fastboot oem get_random_partition 2>&1 | grep bootloader | awk '{print $NF}'`

if [ -z "$platform" ];then
    platform="$default_platform"
fi

if [ -z "$buildtype" ];then
    buildtype="$default_buildtype"
fi

if [ -z "$slot" ];then
    slot="$default_slot"
fi

if [ "$buildtype" = "user" ];then
    echo $secret_key > default_key.bin
    fastboot flash $secret_partition default_key.bin
    fastboot flashing unlock
    fastboot flashing unlock_critical
fi

# function of download
split=$(printf "%-60s" "-")
function flash_one_image() {
    echo -e "\n${split// /-}"
    if [ -e "${platform}_$2" ];then
        echo -e "\E[0;32mbegin fastboot download ${platform}_$2\E[00m\n"
        fastboot flash $1 ${platform}_$2
    elif [ -e "$2" ];then
        echo -e "\E[0;32mbegin fastboot download $2\E[00m\n"
        fastboot flash $1 $2
    else
        echo -e "\E[1;31mCan't find file: $2 or ${platform}_$2, Skip!\E[00m\n"
    fi
}

echo -e "\nAll is download,do you want to reboot(y/n) ?"
read x
if [ "$x" == "y" ] || [ "$x" == "Y" ];then
    fastboot oem recovery_and_reboot
fi

Recently my phone was given to service. I don't know whether they have done anything wrong with my system firmware.