Cyclic reference during evaluation

When you break this down logically, your query says:

Add column named MeseGennaio and then populate it like this:

if [DWH.GENNAIO] = null then do the following: if the month of [Inzio Lavori] is 1 then Merge1[Ricavo pesado]/Month number of [Fine Levori] + 1 -1. //-1 here is your - Date.Month([Inzio Lavori]) value and since you already said (if that value = 1 then do the rest of the calculation) then that value will always be -1.

I would suggest adding parentheses to make sure that your calculations are what you expect. It's difficult to understand if you are trying to divide Ricavo pesado by the month number +1, or by month number +1-1, or by month number, and then adding the integers. Remember, clearly separated in, then, else or elseifs, and clearly delineated parentheses for your calculations are important in if then else. 

--Nate

Cyclical errors occur when a value in a calculation refers to itself, like:

NewTable = Table.Split(NewTable, 2)