Overview:
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.
Root Cause:
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.
Solution:
The child Territories need to be reassigned or deleted, otherwise, the parent Territory cannot be deleted.
Related Documentation:
CRM Help Documentation: Territory Management Overview