Oracle SQL - get previous weekday

Code:

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SELECT
INVOICE.APINVOICE,
UIS_HISTORY.MODIFIED_BY_
to_date(to_char(UIS_HISTORY.DATE_OF_CHANGE, 'MM/DD/YYYY HH:MI:SS AM'),'MM/DD/YYYY HH:MI:SS AM')
FROM
 INVOICE,
 UIS_HISTORY
WHERE
 ( UIS_HISTORY.INVOICE_SYS=INVOICE.INVOICE_SYS_ID )
AND
UIS_HISTORY.DATE_OF_CHANGE=to_date(IF datepart(sysdate,dp_weekday)=1 THEN sysdate-3 ELSE sysdate-1,,'MM/DD/YYYY HH:MI:SS AM')

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keeps saying missing right parenthesis!???

so, do you see where? or is this all wrong

Is it because you have two commas together near the end there? I'm not sure what PL SQL allows but I don't think that would be permitted in T-SQL.