Moving Monthly average

Hi,

trying to compute the monthle moving average of sales.
In my case i have merged two table in the QueryEditor, both fact tables.
Table 1: Containing the facts on invoice number level, and;
Table 2: Containing the facts on invoice line level.
Relation therfore is One (table 1) to Many (Table 2).
I created the merged table as new, called "Merge facturen met mutaties" Dutch for: "Merge invoices with mutations"

I also created a Date table in DAX. Nothing unusal there.
I created a relationship beteween the date table and merged table.
Many (Invoice dates) to One (Date table)

now i would like to compute the monthly moving average of sales over the past 12 months.
I used the syntax (thanks SLQBI) to do so, but unfortunatly it did not work in my case.