Calculating Average Monthly Quantity

Try the following:

1) Create a date table:

Date = 
    VAR EarliestYear = YEAR ( MIN ( Sales[Date] ) )
    VAR LatestYear = YEAR ( MAX ( Sales[Date] ) )

    RETURN
    ADDCOLUMNS (
        CALENDAR (
            DATE ( EarliestYear, 1, 1 ),
            DATE ( LatestYear, 31, 12 )
        ),
        "Month", DATE ( YEAR ( [Date] ), MONTH ( [Date] ), 1 ) //Format as mmmm YYYY
    )

2) Set column types as date for both and format Month as mmmm YYYY

3) Mark it as a date table.

4) Create relationship to your main table (I called mine Sales)

5) Create a measure in the main table:

Average Monthly Quantity = 
    AVERAGEX (
        VALUES ( 'Date'[Month] ),
        CALCULATE ( SUM ( Sales[Quantity] ) )
    )

6) Use it in a table visual: