Rolling Average Time intelligence error

It's because Power BI tries to help people by automatically generating hidden calendar tables that are related to Date columns in the model.  These hidden but separate Date tables are accessed using the dot notation like "'Dim Date'[Date].[Date]" and are required for some Power BI generated Quick Measures.  The quick measure is checking to make sure the user doesn't put a filter on their user created, non-hidden Date column ( 'Dim Date'[Date] in your case ) so the calculation does not return an incorrect result. I always create my own Calendar tables and turn off these hidden calendar tables so not sure if there is a workaround.  Of course, you could also just modify the Quick Measure Dax to remove the dot notation like this: 

Measure =
VAR __LAST_DATE =
    LASTDATE ( 'Dim Date'[Date] )
RETURN
    AVERAGEX (
        DATESBETWEEN (
            'Invoice_Summary All'[Date],
            DATEADD ( __LAST_DATE, -6, DAY ),
            __LAST_DATE
        ),
        CALCULATE ( SUM ( 'Invoice_Summary All'[Sales Per Line] ) )
    )

Quick Measures are a good way to learn DAX coding techniques.

Now that Power BI has the ability to "Mark as Date Table" like in excel, perhaps the Quick Meaures will be updated to use user created Calendar tables(?)