Bootloader Unlock Issue - Sony Xperia M4 Aqua

Hello everyone; I need your help and assistance.
I am having trouble unlocking the bootloader of a Sony Xperia M4 Aqua E2303 device; the Service Menu (*#*#7378423#*#*) reveals Rooting Status: Bootloader unlock allowed - Yes.
Device Details
Android version: 5.0
Build number: 26.1.A.3.111
I have downloaded and installed the corresponding driver from https://developer.sony.com/develop/drivers/, downloaded and installed the Google USB Driver (https://developer.android.com/studio/run/win-usb), and availed an unlock code from https://developer.sony.com/develop/open-devices/get-started/unlock-bootloader/
From hereon, I downloaded and extracted the SDK Platform-Tools for Windows (https://developer.android.com/studio/releases/platform-tools), but utilising the code
Code:
fastboot -i 0x0fce oem unlock 0x<myunlockcode>
(referred from https://developer.sony.com/develop/open-devices/get-started/unlock-bootloader/how-to-unlock-bootloader/) only yielded within Command Prompt the message: fastboot: unknown option -i.
Since the above procedure did not yield the desired result, I attempted the alternative: Flashtool method, using the [Guides][Root][FlashTool][Drivers][Memory Problems][Unlock Bootloader] [TA Backup] (https://forum.xda-developers.com/m4-aqua/general/guide-t3130092) as a reference guide. I downloaded and installed Flashtool 0.9.18.6, and the Flashtool Drivers; albeit the installation directory on my computer is set C:\Flashtool, inside C:\Users\<myusername>\.flashTool\registeredDevices, I created a folder titled mydevices, within it another folder having the device code as its title, and within the latter folder I created a text file titled ulcode, having the unlock code as data entry.
However, post flash mode and fastboot mode processes, Flashtool concludes ....INFO - FAILED (remote: Command not allowed). I have furnished the narrative of the Flashtool process' log below.
08/024/2018 02:24:04 - INFO - Flashtool Version 0.9.18.6 built on 30-05-2015 22:30:00
08/024/2018 02:24:04 - INFO - Executing search strategies to find proxy selector
08/024/2018 02:24:04 - INFO - No proxy found for IE. Trying next one
08/024/2018 02:24:04 - INFO - Strategy firefox failed trying next one : No Firefox installation found
08/024/2018 02:24:04 - INFO - No proxy found for java. Trying next one
08/024/2018 02:24:04 - INFO - Syncing devices from github
08/024/2018 02:24:04 - INFO - Scanning devices folder for changes.
08/024/2018 02:24:11 - INFO - Pulling changes from github.
08/024/2018 02:24:11 - INFO - Devices sync finished.
08/024/2018 02:24:12 - INFO - Device disconnected
08/024/2018 02:24:22 - INFO - Device connected with USB debugging off
08/024/2018 02:24:22 - INFO - For 2011 devices line, be sure you are not in MTP mode
08/024/2018 02:24:22 - INFO - Device connected with USB debugging on
08/024/2018 02:24:22 - ERROR - root : this bundle is not valid
08/024/2018 02:24:23 - INFO - Connected device : Sony Xperia M4 Aqua
08/024/2018 02:24:23 - INFO - Installed version of busybox : N/A
08/024/2018 02:24:23 - INFO - Android version : 5.0 / kernel version : 3.10.49-perf-gc6d4e64 / Build number : 26.1.A.3.111
08/024/2018 02:24:31 - INFO - Please connect your device into flashmode.
08/024/2018 02:24:36 - INFO - Device disconnected
08/025/2018 02:25:22 - INFO - Device connected in flash mode
08/025/2018 02:25:22 - INFO - Opening device for R/W
08/025/2018 02:25:22 - INFO - Reading device information
08/025/2018 02:25:22 - INFO - Phone ready for flashmode operations.
08/025/2018 02:25:22 - INFO - Current device : E2303 - YT9114QVLK - 1293-9369_R16A - 1291-5963_26.1.A.3.111 - GENERIC_26.1.A.3.111
08/025/2018 02:25:22 - ERROR - root : this bundle is not valid
08/025/2018 02:25:22 - INFO - Processing loader.sin
08/025/2018 02:25:22 - INFO - Checking header
08/025/2018 02:25:22 - INFO - Flashing data
08/025/2018 02:25:22 - INFO - Processing of loader.sin finished.
08/025/2018 02:25:23 - INFO - Loader : S1_Root_54c8 - Version : MSM8936_14 / Boot version : S1_Boot_MSM8939_LA2.0_37 / Bootloader status : ROOTABLE
08/025/2018 02:25:23 - INFO - Start Reading unit 000008B2
08/025/2018 02:25:23 - INFO - Reading TA finished.
08/025/2018 02:25:23 - INFO - Ending flash session
08/025/2018 02:25:23 - INFO - Now unplug your device and restart it into fastbootmode
08/025/2018 02:25:23 - INFO - Device connected in flash mode
08/025/2018 02:25:24 - INFO - Device disconnected
08/025/2018 02:25:27 - INFO - Device connected in flash mode
08/025/2018 02:25:33 - INFO - Device disconnected
08/025/2018 02:25:51 - INFO - Device connected in fastboot mode
08/029/2018 02:29:20 - INFO - Unlocking phone using key XXXXXXXXXXXXXXXX
08/029/2018 02:29:20 - INFO - ...
08/029/2018 02:29:20 - INFO - FAILED (remote: Command not allowed)
08/029/2018 02:29:20 - INFO - finished. total time: 0.005s
I await your valuable replies, correcting whatever mistake(s) I may have very likely committed.

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Having done almost all of the important leg work, I overlooked the most fundamental dynamic: Enable OEM unlock within Developer options; I was unaware of the said feature due to lack of sustained experience with Sony Xperia devices!
:silly:
The bootloader has been successfully unlocked, subsequently, custom recovery (TWRP) was successfully flashed, and the device was successfully rooted.
:good: