declare @last6months table (monthId int);
insert @last6months(monthId)
values
    (MONTH(DATEADD(month, -5, GETDATE()))),
    (MONTH(DATEADD(month, -4, GETDATE()))),
    (MONTH(DATEADD(month, -3, GETDATE()))),
    (MONTH(DATEADD(month, -2, GETDATE()))),
    (MONTH(DATEADD(month, -1, GETDATE()))),
    (MONTH(GETDATE()))
;

SELECT
    ISNULL(daysAvg.PickPerStoreMonthly, 0) PickPerStoreMonthly,
    last6months.monthId
FROM
    (select * from @last6months) last6months
LEFT JOIN (
    SELECT
        AVG (DATEDIFF ( day, item.CreateDate , point.Arrived)) PickPerStoreMonthly,
        MONTH(point.Arrived) monthId
    FROM
        Location location WITH (NOLOCK)
        LEFT JOIN RoutePoint point WITH (NOLOCK) ON point.LocationId = location.Id
        LEFT JOIN Route route WITH (NOLOCK) ON route.Id = point.RouteId
        LEFT JOIN Item item WITH (NOLOCK) ON item.RouteId = route.Id

    WHERE
        point.Arrived > DATEADD(month, -5, GETDATE())
        AND location.Id = 18
    GROUP BY (MONTH(point.Arrived))
) daysAvg ON daysAvg.monthId = last6months.monthId
;