Are you sure you're navigating to the LDAP settings correctly? This needs to be done by microflow, not by directly linking to the overview page.
The 'user entity' dropdown in the server configuration form is empty.
Probably you were navigation to the Ldap.LdapServers_Overview form directly instead of using the Ldap.OpenLdapServersOverview microflow in your navigation. This microflow makes sure the domain model is analyzed before opening the appropriate forms.
See also the documentation at https://appstore.home.mendix.com/link/app/24/Mendix/LDAP-Synchronization-module under Common errors