Adding missing date rows and filling in Quantity

I think this is because CALENDAR generates a table with a column named [Date], not [Inv_Date].

Try using [Date] instead of [Inv_Date] inside of EARLIER or else use variables like this:

FullDateTable =
ADDCOLUMNS (
    CALENDAR (
        MIN ( INVENTORY[Inv_Date] ),
        MAX ( INVENTORY[Inv_Date] )
    ),
    "Quantity",
        VAR CurrDate = [Date]
        VAR LastInv_Date =
            MAXX (
                FILTER (
                    INVENTORY,
                    INVENTORY[Inv_Date] <= CurrDate
                ),
                [Inv_Date]
            )
        RETURN
            LOOKUPVALUE (
                INVENTORY[Quantity],
                INVENTORY[Inv_Date], LastInv_Date
            )
)