did you run with sudo?
sudo fastboot devices
^^ does it show up? if yes then
sudo fastboot oem rebootRUU
^^ wait a second or two
sudo fastboot flash zip 1.29.401.12_hboot_1.44.zip
copy/paste terminal output
ubuntu@ubuntu:~$ sudo fastboot devices
sudo: fastboot: command not found
I don't know why this is so. I installed fastboot so many times.