Can someone explain me what this error means?

Priyanshu1325 It's not an error. Excel merely alerts you to the fact that you perhaps missed some cells in the formula. The header row shows "Jan-2019" but probably is a real date, hence a number. The formula sums B4 to B8 and Excel's background error checking thinks that you perhaps also want to include the number from B3. Obviously not, so you can ignore the alert.