fastboot devices Again, your device serial appears. Good. Most Nokia phones would stop here, showing fastboot oem unlock as “not allowed”. But your Nokia 3.4 has a secret: an engineering bootloader exploit.
Now reboot to bootloader:
Run it (right-click → Run as administrator). Unlock Bootloader Nokia 3.4
On your PC, open a command prompt in the platform-tools folder and type: fastboot devices Again, your device serial appears