Before registering Nebula devices, you have to create organization and site first, and you can refer the following steps.
1. Go to Organization > Create organization.
2. You can either use the Wizard for the next steps:
or skip the Wizard and enter "Organization name," and click "Create organization."
3. Go to Organization > Create site.
4. Enter "Site name," and click "Create site."
You can now go ahead and add respective devices into your Nebula account according to this article: How to register a product in Nebula Control Center