Fastboot error encountered when attempting to unlock bootloader of my Xperia 10 III

[XperiaFlashaholic]

Two attempts were made to unlock the bootloader of my Xperia 10 III(XQ-BT52) in order to install "Software binaries for AOSP Android 11.0 – Kernel 4.19 – Lena (latest)" via Ubuntu and Windows 10 respectively. The official guide on unlocking the bootloader of Xperia devices was followed entirely in the Windows 10 attempt, though source for the Platform tools differed in the Ubuntu attempt.

Both attempts resulted in an identical error when executing the final command:

fastboot oem unlock 0xXXXXXXXXXXXXXXX
...
FAILED (remote: Command not supported in default implementation)
finished. total time: 0.00

Below is record of the process from initial setup to the final error during the Ubuntu attempt:

ubuntu@ubuntu:~$ sudo apt-get install android-tools-adb
Reading package lists... Done
Building dependency tree 
Reading state information... Done
The following additional packages will be installed:
adb android-libadb android-libbase android-libboringssl
android-libcrypto-utils android-libcutils android-liblog
android-sdk-platform-tools-common
The following NEW packages will be installed:
adb android-libadb android-libbase android-libboringssl
android-libcrypto-utils android-libcutils android-liblog
android-sdk-platform-tools-common android-tools-adb
0 upgraded, 9 newly installed, 0 to remove and 163 not upgraded.
Need to get 862 kB of archives.
After this operation, 2707 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://archive.ubuntu.com/ubuntu focal/universe amd64 android-liblog amd64 1:8.1.0+r23-5ubuntu2 [41.9 kB]
Get:2 http://archive.ubuntu.com/ubuntu focal/universe amd64 android-libbase amd64 1:8.1.0+r23-5ubuntu2 [22.4 kB]
Get:3 http://archive.ubuntu.com/ubuntu focal/universe amd64 android-libboringssl amd64 8.1.0+r23-2build1 [538 kB]
Get:4 http://archive.ubuntu.com/ubuntu focal/universe amd64 android-libcrypto-utils amd64 1:8.1.0+r23-5ubuntu2 [8324 B]
Get:5 http://archive.ubuntu.com/ubuntu focal/universe amd64 android-libcutils amd64 1:8.1.0+r23-5ubuntu2 [22.5 kB]
Get:6 http://archive.ubuntu.com/ubuntu focal/universe amd64 android-libadb amd64 1:8.1.0+r23-5ubuntu2 [123 kB]
Get:7 http://archive.ubuntu.com/ubuntu focal/universe amd64 adb amd64 1:8.1.0+r23-5ubuntu2 [87.6 kB]
Get:8 http://archive.ubuntu.com/ubuntu focal/universe amd64 android-sdk-platform-tools-common all 27.0.0+12 [6648 B]
Get:9 http://archive.ubuntu.com/ubuntu focal/universe amd64 android-tools-adb all 1:8.1.0+r23-5ubuntu2 [11.0 kB]
Fetched 862 kB in 3s (299 kB/s) 
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Unpacking android-libboringssl (8.1.0+r23-2build1) ...
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Unpacking android-libcrypto-utils (1:8.1.0+r23-5ubuntu2) ...
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Unpacking android-libcutils (1:8.1.0+r23-5ubuntu2) ...
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Selecting previously unselected package adb.
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Unpacking adb (1:8.1.0+r23-5ubuntu2) ...
Selecting previously unselected package android-sdk-platform-tools-common.
Preparing to unpack .../7-android-sdk-platform-tools-common_27.0.0+12_all.deb ...
Unpacking android-sdk-platform-tools-common (27.0.0+12) ...
Selecting previously unselected package android-tools-adb.
Preparing to unpack .../8-android-tools-adb_1%3a8.1.0+r23-5ubuntu2_all.deb ...
Unpacking android-tools-adb (1:8.1.0+r23-5ubuntu2) ...
Setting up android-sdk-platform-tools-common (27.0.0+12) ...
Setting up android-liblog (1:8.1.0+r23-5ubuntu2) ...
Setting up android-libboringssl (8.1.0+r23-2build1) ...
Setting up android-libcrypto-utils (1:8.1.0+r23-5ubuntu2) ...
Setting up android-libbase (1:8.1.0+r23-5ubuntu2) ...
Setting up android-libcutils (1:8.1.0+r23-5ubuntu2) ...
Setting up android-libadb (1:8.1.0+r23-5ubuntu2) ...
Setting up adb (1:8.1.0+r23-5ubuntu2) ...
Setting up android-tools-adb (1:8.1.0+r23-5ubuntu2) ...
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for libc-bin (2.31-0ubuntu9.2) ...
ubuntu@ubuntu:~$ sudo apt-get install android-tools-fastboot
Reading package lists... Done
Building dependency tree 
Reading state information... Done
The following additional packages will be installed:
android-libbacktrace android-libetc1 android-libf2fs-utils android-libsparse
android-libunwind android-libutils android-libziparchive
android-sdk-platform-tools dmtracedump etc1tool f2fs-tools fastboot graphviz
hprof-conv libann0 libcdt5 libcgraph6 libf2fs-format4 libf2fs5 libgts-0.7-5
libgts-bin libgvc6 libgvpr2 liblab-gamut1 libpathplan4 p7zip p7zip-full
sqlite3
Suggested packages:
gsfonts graphviz-doc p7zip-rar sqlite3-doc
The following NEW packages will be installed:
android-libbacktrace android-libetc1 android-libf2fs-utils android-libsparse
android-libunwind android-libutils android-libziparchive
android-sdk-platform-tools android-tools-fastboot dmtracedump etc1tool
f2fs-tools fastboot graphviz hprof-conv libann0 libcdt5 libcgraph6
libf2fs-format4 libf2fs5 libgts-0.7-5 libgts-bin libgvc6 libgvpr2
liblab-gamut1 libpathplan4 p7zip p7zip-full sqlite3
0 upgraded, 29 newly installed, 0 to remove and 163 not upgraded.
Need to get 4806 kB of archives.
After this operation, 19.6 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
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Setting up fastboot (1:8.1.0+r23-5ubuntu2) ...
Setting up android-sdk-platform-tools (27.0.0+12) ...
Setting up android-tools-fastboot (1:8.1.0+r23-5ubuntu2) ...
Processing triggers for libc-bin (2.31-0ubuntu9.2) ...
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for initramfs-tools (0.136ubuntu6.6) ...
update-initramfs is disabled since running on read-only media
ubuntu@ubuntu:~$ adb devices
List of devices attached
* daemon not running; starting now at tcp:5037
* daemon started successfully

ubuntu@ubuntu:~$ adb devices
List of devices attached
XXXXXXXXXX device

ubuntu@ubuntu:~$ adb reboot bootloader
ubuntu@ubuntu:~$ fastboot devices
XXXXXXXXXX fastboot

ubuntu@ubuntu:~$ fastboot reboot
rebooting...

finished. total time: 0.050s
ubuntu@ubuntu:~$ adb devices
List of devices attached
XXXXXXXXXX device

ubuntu@ubuntu:~$ adb reboot fastboot
ubuntu@ubuntu:~$ fastboot devices
XXXXXXXXXX fastboot
ubuntu@ubuntu:~$ fastboot oem unlock 0xXXXXXXXXXXXXXXX
...
FAILED (remote: Command not supported in default implementation)
finished. total time: 0.000s

Any and all assistance would be greatly appreciated.

[MarlonUk1]

Hello @XperiaFlashaholic 

Have you tried to see if there solutions provider at XDA forums?