Private Temporary Tables (PTT) is a new feature in Oracle 18c, they have the similar characteristics of global temp tables but instead of being created under the TEMPORARY tablespace, they get created in the session's PGA, which is a great feature for boosting the performance (if your PGA is big enough to host this kind of PTT's).
SQL> CREATE PRIVATE TEMPORARY TABLE ORA$PTT_users ON COMMIT PRESERVE DEFINITION AS SELECT * FROM dba_users;
CREATE PRIVATE TEMPORARY TABLE ORA$PTT_users ON COMMIT PRESERVE DEFINITION AS SELECT * FROM dba_users
ERROR at line 1:
ORA-14451: unsupported feature with temporary table
SQL> sho user
USER is "SYS"
Above misleading error will pop-up if you create a Private Temporary Table (PTT) while connecting as SYS, SYSDBA, SYSRAC or SYSBACKUP.
Create the PRIVATE TEMPORARY TABLE (PTT) with any other user not connecting as SYSDBA, SYSRAC, SYSBACKUP,... etc.