Can you verify what is in the data item [fileNm]? There should be no difference between passing it in a data item and passing it as a string literal in the input.
Data Item Value should be like this with no quotes: C:\Users\default\Documents\PerformanceTimings.xlsx
If you put it in the input directly then it looks like this: "
C:\Users\default\Documents\PerformanceTimings.xlsx"
And if you use the Data Item instead then like this, just as you have: [fileNm]------------------------------
Dave Morris
3Ci @ Southern Company
Atlanta, GA
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Dave Morris, 3Ci at Southern Company