Actually you're going a bit too far this time.
You don't want to pass strings to the Format into String VI that have already been formatted with the Array to Spreadsheet VI. Just format the header info with Format into String, format the data with Array to Spreadsheet, THEN concatenate them together with Concatenate String. Just like in my example
you may have to add an extra TAB or NEWLINE to get it right. Create a VI like my example to test out changes you make and it will become clear. Don't feel bad, string manipulation is a pain in most languages.
LabVIEW Pro Dev & Measurement Studio Pro (VS Pro) 2019