On older devices (typically pre-2015), you might need to use the legacy command fastboot oem unlock instead [12†L22][12†L33]. This command also initiates the unlock process and will wipe your data.
Hold the buttons simultaneously until you see the Fastboot/Bootloader screen (often depicted by a fastboot logo or text). On older devices (typically pre-2015), you might need
Connect your phone to your computer via a reliable USB data cable. On older devices (typically pre-2015)
Go to Settings > About Phone and tap "Build Number" 7 times. On older devices (typically pre-2015), you might need
Tap on seven times until "You are now a developer" appears. Go to Settings > System > Developer Options . Locate and toggle on OEM Unlocking and USB Debugging .