Oracle 18c

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select TO_CHAR(sysdate, 'DD-MON-YYYY HH24:MI:SSxFF') from dual;
ORA-01821: date format not recognized

select TO_CHAR(sysdate, 'DD-MON-YYYY HH24:MI:SSxFF3') from dual;
ORA-01821: date format not recognized

select TO_CHAR(sysdate, 'DD-MON-YYYY HH24:MI:SS') from dual;

TO_CHAR(SYSDATE,'DD-MON-YYYYHH24:MI:SS')

14-MAY-2020 06:03:03

select TO_CHAR(CAST(sysdate AS TIMESTAMP), 'DD-MON-YYYY HH24:MI:SSxFF') from dual;

TO_CHAR(CAST(SYSDATEASTIMESTAMP),'DD-MON-YYYYHH24:MI:SSXFF')

14-MAY-2020 06:03:03.000000

select TO_CHAR(CAST(sysdate AS TIMESTAMP), 'DD-MON-YYYY HH24:MI:SSxFF3') from dual;

TO_CHAR(CAST(SYSDATEASTIMESTAMP),'DD-MON-YYYYHH24:MI:SSXFF3')

14-MAY-2020 06:03:03.000

select TO_CHAR(systimestamp, 'DD-MON-YYYY HH24:MI:SSxFF') from dual;

TO_CHAR(SYSTIMESTAMP,'DD-MON-YYYYHH24:MI:SSXFF')

14-MAY-2020 06:03:03.906859