I am trying to enable "Manage Territories" user permission in a profile but there is no option for me to. Is there a reason why that permission is not present?
Only certain licenses are allowed to access territories (and thus manage them). If you're editing permissions for a profile for a license that doesn't have access to territories then you can't enable territory management.
E.g, a profile associated with a full Salesforce license will have the option but a profile associated with a Force.com - App Subscription license will not.
The other condition under which the permission would not be available is if territory management was not enabled. Enable in
Manage Territories > Settings in setup.