Formula wont work.
Comments

It looks like you may have an excel tab reference 'Combined Timesheets'! in the formula. Can you write out in words what you want the formula to do?
If Column2 Row5 equals a comma,
Then
(value if true)
(value if false)
If you have more conditions to evaluate just indent as needed. I think this will lead to a better structured formula in the end (using index/match, OR, @row). Looking forward to your response.
=IF(
[Column2]5="","",
(IF
((VLOOKUP([Column4]5,'Combined Timesheets'!A:
[Column2],2,FALSE))=[Column2]5,"Yes",""
))
) 
There are several potential issues I see with this formula... which makes the final one notneeded. Also there is what looks to be an excel or Google sheet reference to an alternative
First there is an extra open parenthesis before the 2nd IF statement
=IF([Column2]5="","",> (<IF((VLOOKUP([Column4]5,'Combined Timesheets'!A:[Column2],2,FALSE))=[Column2]5,"Yes",""))>)<
Second, it looks like there is a reference to a separate google sheet that might have come when you ported this over from Exel or Google. This could also e causing the problem.
=IF([Column2]5="","",IF((VLOOKUP([Column4]5,>'Combined Timesheets'!A:<[Column2],2,FALSE))=[Column2]5,"Yes",""))

Thanks for your reply,
I imported several tabs from an excel workbook and many of them contain referencing formulas. I imported the formulas as well but its not working in smartsheet.

Referencing another sheet in Smartsheet is VERY different than referencing another sheet in Excel.
Here is an article from Smartsheet on how to reference other sheets...
https://help.smartsheet.com/articles/2476606formulasreferencedatafromothersheets
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