SQL: Grouping on a subquery - sql

Question

I have a query where I want to aggregate column ElementAantal. However this is the first time I am grouping on columns where there is a subquery ArtikelInhoud involved.
Please your advise on how to make the query work.
SELECT
pl.[Location Code] AS Locatie,
pl.[Ending Date] AS ProdOrderDatum,
pl.[Item No_] AS Artikelnr,
pl.Description AS Artikel,
pbl.No_ AS Elementnr,
pbl.Description AS Element,
SUM(pbl.Quantity * pl.Quantity) AS ElementAantal, -----> Aggregating this column
i.[Item Category Code] AS ElementCategorie,
(
SELECT Value
FROM [$Attribute Specification] att
WHERE [Table ID] = 27 AND Code = 'INH' AND att.[Unique Record Identifier] = i1.[Unique Record Identifier]
) AS ArtikelInhoud,
i.[Gross Weight] AS ElementGewicht,
v.Name AS Leverancier
FROM [$Prod_ Order Line] pl
LEFT JOIN [$Production BOM Line] pbl ON pl.[Production BOM No_] = pbl.[Production BOM No_]
LEFT JOIN [$Item] i ON pbl.No_ = i.No_
LEFT JOIN [$Item] i1 ON pl.[Item No_] = i1.No_
LEFT JOIN [$Vendor] v ON i.[Vendor No_] = v.No_
GROUP BY pl.[Location Code],
pl.[Ending Date],
pl.[Item No_],
pl.Description,
pbl.No_,
pbl.Description,
i.[Item Category Code],
(
SELECT Value
FROM [$Attribute Specification] att
WHERE [Table ID] = 27 AND Code = 'INH' AND att.[Unique Record Identifier] = i1.[Unique Record Identifier]
),
i.[Gross Weight],
v.Name