Most likely you have added an external LDAP (or Crowd) user directory, and then disabled the built-in "Internal" user directory.
What do you actually want to do? Add a user to LDAP (Crowd) ? then contact the LDAP admin.
Add a user to the Internal directory? Then you need to re-activate it.
... or add some more details to the question
We’re excited to introduce external collaboration for Confluence, now available in early access. It is available to preview for Confluence Cloud Premium and Enterprise customers. (If you're not on ...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events