If Excel is already running by the time you want to run this with the target file open, you can't just start a new excel instance and open that workbook; it will just create a new file with the same name, and when it comes time to save, you will get the error that the file is already open (anyway that's where I encountered error when I ran your example).
Instead, you need to fetch the open Excel instance
Excel = actxGetRunningServer('Excel.Application')
If you have multiple workbooks open in the excel instance, you'll need to find the one that you want within the list Workbooks.Item
Excel.Workbooks.Item(k).FullName
Such as by looping through and checking its FullName (full path, I think)
