Our desired result is to enforce SSO for our organization for all of our cloud and SaaS applications. I inherited our Confluence Cloud (Standard) environment and the person who configured the system is no longer with our organization.
I've confirmed SSO is configured and working correctly, as I've been able to add our non-SSO users to the AD group we use for access and can see them appear as users in user administration.
In Confluence, when I go to Settings --> Security Configuration --> Edit Security and Privacy, the checkbox for External user management remains disabled and I cannot make the desired change. (See image below)

What do I need to do so that:
1. I can enable external user management?
2. SSO is the only way our users can authenticate in Confluence?
I'm experienced with Jira administration, but new to Confluence both as a user and an admin.