Я думаю, что проблема в неуместных скобках
SELECT a.WeekNo,
a.EssbaseCustomer,
a.SKU,
a.SKU_Description,
c.[Brand],
d.Index,
a.Promo_Kg,
a.Promo_NSV,
b.NonPromo_NSVkg,
a.Promo_NSV/a.Promo_kg AS Promo_NSVkg
FROM ((Actuals AS a
LEFT JOIN PriceList AS b ON (a.SKU=b.SKU) AND (a.EssbaseCustomer=b.EssbaseCustomer))
LEFT JOIN Map_SKU AS c ON a.SKU=c.SKU
LEFT JOIN Seasonality AS d ON a.Weekno=d.Weekno AND d.brand=c.Brand)
WHERE a.Promo_Kg>0;
Или еще лучше, без скобок:
SELECT a.WeekNo,
a.EssbaseCustomer,
a.SKU,
a.SKU_Description,
c.[Brand],
d.Index,
a.Promo_Kg,
a.Promo_NSV,
b.NonPromo_NSVkg,
a.Promo_NSV/a.Promo_kg AS Promo_NSVkg
FROM Actuals AS a
LEFT JOIN PriceList AS b ON a.SKU=b.SKU AND a.EssbaseCustomer=b.EssbaseCustomer
LEFT JOIN Map_SKU AS c ON a.SKU=c.SKU
LEFT JOIN Seasonality AS d ON a.Weekno=d.Weekno AND d.brand=c.Brand
WHERE a.Promo_Kg>0;
Надеюсь, это поможет ...