Before the migration from IOP 4.2.5 to HDP 2.6.4, we used PAM authentication for Ranger Admin.
Now we can only have UNIX, LDAP or AD.
Why ? it's very useful to use PAM authentication.
AMBARI-18425 will be available in Ambari 2.7.0 (to be released).
In the meanwhile, if using Ambari 2.6.x (with HDP 2.6.x), set the authentication method as UNIX and add a custom property ranger.pam.authentication.enabled in ranger-admin-site.xml and set it to true.