Can't unlock bootloader - fastboot issue

Hello,

I am trying to unlock my bootloader but I my computer (Windows 10) does not recognize fastboot.
When I connect to fastboot (holding volume up button + connecting my phone by USB when it's off, or using the adb reboot fastboot command), the computer does not recognize the phone. I then tried installing drivers (both from Google and Sony) by going into device manager but Windows just doesn't install the drivers.

I then saw this on Sony's website:
[URL...

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