OpenLDAP create new Organization & Organization Unit using Apache Directory Studio LDAP Browser

I have set up an OpenLDAP server via slapd in macOS 10.15.7. I successfully connected to the server in Apache Directory Studio LDAP Browser. However, when I try to add any new entry to the server, the same error exists:

[LDAP result code 53 - unwillingToPerform] no global superior knowledge

Here is a screenshot when I am trying to create an OU:

It said the Parent does not exist. When I leave Parent empty, it said "no global superior knowledge". So what should I do if I want to set up the LDAP server with new OU, O and CN?