Compartments are the primary building blocks you use to organize your cloud resources. You use compartments to organize and isolate your resources to make it easier to manage and secure access to them.
Compartments are tenancy-wide, across regions. When you create a compartment, it is available in every region that your tenancy is subscribed to. You can get a cross-region view of your resources in a specific compartment with the compartment explorer.
When your tenancy is provisioned, a root compartment is created for you. Your root compartment holds all of your cloud resources. You can think of the root compartment like a root folder in a file system.
To delete a compartment, it must be empty of all resources. Before you initiate deleting a compartment, be sure that all its resources have been moved, deleted, or terminated, including any policies attached to the compartment.
so Answer "make sure you have at least one more compartment in your tenancy other than the root compartment." which you can keep only the root compartment and not mandatory to have other compartment