Since you didn't attach the subVI Set Cell Value, it's impossible to fix but one thing I think you should do is keep the same references and pass them and use dataflow with the error in/error out connections. I'm refering to that little sequence you have at the bottom. Why not just pass the workbooks ref that you already have and then pass the sheets ref to the subVI. Also, don't close the sheets ref before you pass it to the subVI. You need to keep it open for the subVI to work.
