I'm not going to pretend my C# is great (it's not, I use VB.net myself), but I notice there there do appear to be calls to Open the workbook. I can't see it anywhere, but if this is the case wouldn't this "Open" Excel and load the Workbook inside that? Thus, created a graphical interface? Infact, I can see references to Worksheet.Activate, which heavily suggests this.

If this is the case, my suggestion from further up the topic seems to be correct. you're trying to do something graphical in a non-graphical environment. you can't "Open" Excel in an SSIS Package (yes, it let's you in SSDT, as you're running Visual Studio Graphcially). Trying to open a graphical program is like trying to open one in a command line only interface; it doesn't work.

I do have to ask, why are you editing the data after creating it from SSIS anyway? Ideally, you should be exporting the data in the format you require in the first step; then there is no need to play around with the workbook after you're done.