Nokia (HMD Global) updated the bootloader with the August 2018 Android security patch. Updated phones do not accept the bootloader unlock keys.
But it is possible to downgrade the bootloader on these phones. Once downgraded, the device will accept the unlock key and it can be unlocked. You can update the device again once it is unlocked. It will not be automatically re-locked.
Check which security patch is installed on your phone
- On your phone, open Settings > System > About phone.
- Check the date underneath the heading – Android Security Patch.
- If it is August 1, 2018 (or anything newer than that), then your device is updated to the new security.