ORA 1843 not a valid month

I need to format the three dates in three different ways.
I am getting not valid month error.
Can you help? Thanks! Beth

SELECT 
PACKLIST.PACKLIST_NO, 
to_date(TRANSACTION_DATE, 'dd-mm-yyyy'), 
to_date(SCHEDULED_SHIP_DATE, 'mm-dd-yyyy'), 
to_date(ACTUAL_SHIPPED_DATE, 'mm-dd-yy'),
JOBCOST_UNITS * COST_PER_UNIT AS EXTENDED_COST
 
FROM PACKLIST, JOBCOST
 
WHERE PACKLIST.PACKLIST_NO = JOBCOST.PACKLIST_NO;

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