Adding missing date rows and filling in Quantity

It sounds like you're trying to define a measure or calculated column instead of a new table.

As a separate note, you may want to filter the calendar part if you only want one day per month. I.e.

FILTER (
        CALENDAR (
            MIN ( INVENTORY[Inv_Date] ),
            MAX ( INVENTORY[Inv_Date] )
        ),
        DAY ( [Date] ) = 1
    )