Cannot boot to bootloader mode

Hello guys. I really need soem help on this. After trying a whole day and looking at plenty of sources (heck even other phone forums), I've failed to unlock my bootloader. I've tried running the command

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adb reboot bootloader

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and failed countless times. Any suggestions? Also, yes I've selected USB debugging and OEM unlock in developer options. Thank you.

Phone model: LG H960A
OS: Android 6.0