It's difficult understand full story with this screenshot.
If you can attach your packaged workbook(***.twbx), that is most helpful.
HI Mauricio, I agree with Shin, it's tough to figure out what's going on without the packaged workbook. Are you trying to do a rolling sum of the last 7 months? If so, you can do that easily using a Quick Table Calculation, or you could also try this:
WINDOW_SUM(ZN(Sum(ZN([cfVentas]))), MAX(-6, FIRST()), 0)