Locked the bootloader and "fastboot oem unlock" not working

 Hello. Managed to lock the bootloader and cannot find a way to unlock it. I've tried to boot the device into bootloader and command "fastboot oem unlock" returns "OKAY" but the bootloader is still locked and returns "FAILED (remote: 'device is locked')" . There's also no "Enable OEM unlocking" in developer options.

Did anyone else had their bootloader locked and did you manage to unlock it?