IF function returning error

[rehan0129]

I have two sheets both having similar functionality but in one sheet the IF function works correctly whereas in the other sheet its returning value error even though the syntax writing in both the sheet is same.
Check cell I20 in worksheet [Tax Invoice] of workbook [service invoice tracker] and cell J14 in worksheet [Invoice] of workbook [sales invoice tracker].
In both the cells of two worksheet the formula syntax is same but results are different.
Where is the problem?

[finalazy]

Sorry my mistake. The reason why Invoice_No is not working because there's no value in it.

Just change your formula to
=IF(InvoiceDetail[Invoice No]=InvoiceList,"OK") instead of equal to Invoice_No.
Then ctrl shift enter

The Invoicelist is referring to the drop down invoice.