Unlocking the Nexus 9 bootloader I'm trying to flash the Android M Developer Preview onto my Nexus 9 (volantis) following the steps from Google.

However, when executing the unlock command, I get an error that permission is denied:

$ fastboot oem unlock
...
(bootloader) ability is 0
(bootloader) Permission denied for this command!
OKAY [  0.006s]
finished. total time: 0.006s


What have I done wrong?
The instructions don't make mention of the setting in the device's developer options.

Make sure to enable the "OEM unlocking" setting there: