"nexus-fastboot oem unlock" fails with Nexus 7 2012 wifi

Hi All,

I follow instructions for installing NetHunter at http://www.nethunter.com/prepare/

I get to the "nexus-fastboot oem unlock" and the Nexus 7 screen just shows the yellow android with "start" in the top right corner.

Kali recognises the Nexus 7;

[ 6504.229818] usb 2-1: new high-speed USB device number 41 using ehci-pci
[ 6504.671796] usb 2-1: New USB device found, idVendor=18d1, idProduct=4e40
[ 6504.671803] usb 2-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 6504.671807] usb 2-1: Product: Android
[ 6504.671809] usb 2-1: Manufacturer: Google, Inc
[ 6504.671812] usb 2-1: SerialNumber:

and

nexus-adb devices
List of devices attached

Shows nothing!

Any help and guidance appreciated. MTIA.