Unable to delete Salesforce territory record, the error message is the following: You cannot delete the territory because at least one territory rolls up to it.
The Territory to be deleted has at least one child Territory assigned which prevents the deletion. An error occurs: You cannot delete the territory because at least one territory rolls up to it.
The child Territories need to be reassigned or deleted, otherwise, the parent Territory cannot be deleted.
CRM Help Documentation: Territory Management Overview