Why is no site id generated in the Enphase Installer Toolkit when I am offline?

Site id is generated only when the system is created in the Enlighten. When you create a new system in the Enphase Installer toolkit, the details of the system will be stored locally on your device and it will be synched with the Enlighten only when you connect to the internet. That's why you see a system without a site id when you are offline.