Tldr: Use the principal names from the following query in the delete_application_principal procedure:

SELECT name principal FROM xdb.app_users_and_roles WHERE name <> 'AUTHENTICATED' ORDER BY 1;

Having encountered the same situation while preparing to upgrade from 18c to 19c, and having found no help here or elsewhere as to how to hunt down user/role name(s) in need of the delete_application_principal treatment, I finally thought to look at the code Oracle uses to determine whether or not the identified manual fix-ups have been satisfied.

Looking through the generated preupgrade_fixups.sql led me to the dbms_preup package in the SYS schema, which thankfully wasn't wrapped, and in there I found the function appl_principal_check. In that function was some simple PL/SQL code to count the number of records in two XDB schema views, XDB.APP_USERS_AND_ROLES and XDB.APP_ROLE_MEMBERSHIP. Querying the first of those (and noting the comment "Ignore the AUTHENTICATED record inserted at table creation") gave me a single result (other than the ignored AUTHENTICATED result): 'AUTHORIZED'. After running delete_application_principal with 'AUTHORIZED' as the argument, preupgrade_fixups.sql reported that the app_principal preupgrade check had been remedied.

Note: My situation was that I had (as far as I know) no user-defined applications using XDB or any security defined using "application principals" in my database, so the AUTHORIZED role is possibly a system-generated default (and if so, I would think the pre-upgrade scripts could auto-fix in the case that it's all there is). If you have XDB usage / application principals with users placed in roles, etc., you might have records in XDB.APP_ROLE_MEMBERSHIP, which apparently contains GUIDs rather than names, so you'll possibly have to chase things down further if you do.

So the following code should remedy this in simple cases:

SET SERVEROUTPUT ON BEGIN   FOR r IN (SELECT name principal FROM xdb.app_users_and_roles WHERE name <> 'AUTHENTICATED' ORDER BY 1) LOOP     dbms_output.put_line(r.principal ||' ...');     IF xdb.dbms_xdbz.delete_application_principal(name => r.principal) THEN        dbms_output.put_line('Succeeded.');     ELSE         dbms_output.put_line('Failed.');     END IF;   END LOOP; END; / 

So now there is a solution findable on the 'Net!

All the best with your upgrades, everyone!

Eldon