Error while unlocking bootloader | FAILED (remote: 'Unlock failed - Err:0xffffffff')

I was trying to unlock the bootloader of an old Redmi Note 4x, Downloaded fastboot and adb.I first enabled developer option and turned on USB debugging. I tried finding OEM unlock option which was not present on the phone. After few hours of searching about it online, I finally concluded it is not available for the phone. So I went on and fired up my terminal [in administrator mode];

$ adb devices> List of devices attached> 8H4DZP69S44DQOEM        device

then I rebooted my device in bootloader (fastboot)

$ adb reboot bootloader

My Phone restarted in bootloader, then I went back to my terminal and checked if the fastboot was working or not;

$ fastboot devices> 8H4DZP69S44DQOEM        fastboot

After confirming that the fastboot is running, I tried unlocking OEM;

$ fastboot oem unlock> (bootloader) Start unlock flow> FAILED (remote: '> Unlock failed - Err:0xffffffff> ')> fastboot: error: Command failed

which.. well... failed and I don't know why. I tried searching this very error code online but failed to find anything close to it.

My other attempts like unlocking flashing failed in similar fashion;

$ fastboot flashing unlock> (bootloader) Start unlock flow> FAILED (remote: '> Unlock failed - Err:0xffffffff> ')> fastboot: error: Command failed

$ fastboot flashing unlock_critical> (bootloader) Start unlock flow> FAILED (remote: '> Unlock failed - Err:0xffffffff> ')> fastboot: error: Command failed

Nothing seems to be working, Please help!