Fastboot oem get_identifier_token - FAILED (remote: unknown command) Hello Everyone!

I am trying yo unlock the bootloader with no success. Basically, the following command
Code:

Fastboot oem get_identifier_token


is hitting the error of: 

FAILED (remote: unknown command)


Please, your help will be greately appreciated. Thanks in advance for reading my question.



Device State:



OEM unlocking: is enabled from the developers options.
USB Debugging: is enabled from the developers options.

Software Status: Official
Software State: LOCKED
Software Security: S-ON                                

Device Information:
Model Number: HTC One M9
Android Version: 5.1
HTC Sense Version: 7.0
Software Number: 2.10.401.1
Baseband Version: 01.04_U11440601_71.02.50709G_F
Build Number: 2.10.401.1 CL576582 release-keys
Kernel Version:
3.10.49-perf-g901e7f1
and@AABM #1
SMP PREEMPT
I found the answer to my question at last after hours of researching.

We should execute the command from the fastboot mode (FASTBOOT USB) and not from the bootloader mode (HBOOT). This is a non-default option selectable using the volume buttons in bootloader (HBOOT) mode.That's why it was not working for me.