Have a look at the application.
Hope this is what you want.
variable_field.qvw 170.0 K
In Variable $(ROT1) you have written =Month(MonthStart(Today(), -2))
But there is no Date field in the Excel Spreadsheet and you have no Calendar created in the Script.
So I don't believe these Variables will Work.
Try changing the variables to the following
$(ROT1) as =SUM(ROT1)
$(ROT2) as =SUM(ROT2)
Then try the Expression sum_var like this =$(ROT1) + $(ROT2) and check if you get the correct result.
You will need to format COLUMN C and COLUMN F in the Excel spreadsheet as Numbers and RELOAD the Data
Hope this helps