You'll need to specify the full path of the file or the relative path in regards to where the executable is running. So you can set targetFileName = ".\template.xlsx
another way to get the file is to mark the Build Action in the properties of the file and set it to Embedded Resource. then use the following code to get the stream. Not sure if the stream will help you or not.
Assembly asm = Assembly.GetExecutingAssembly();
string file = string.Format("{0}.Template.xlsx", asm.GetName().Name);
var ms = new MemoryStream();
Stream fileStream = asm.GetManifestResourceStream(file);