Looks like the os x sierra does not allow installer to use Fastboot.
Let's force it.
Just open terminal and type
sudo open -a ubports-installer
This will give proper privileges to the installer.
After that install as usually

Looks like the os x sierra does not allow installer to use Fastboot.
Let's force it.
Just open terminal and type
sudo open -a ubports-installer
This will give proper privileges to the installer.
After that install as usually