SQL Server 2014

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

WITH TestTable (Month, Branch, Supplier, Article, SalesQty, StockQty) AS (
SELECT 201811 AS Month, 345 AS Branch, 1 AS Supplier, 1234 AS Article, 2 AS SalesQty, 10 AS StockQty UNION
SELECT 201812 AS Month, 345 AS Branch, 1 AS Supplier, 1234 AS Article, 3 AS SalesQty, 12 AS StockQty UNION
SELECT 201901 AS Month, 345 AS Branch, 1 AS Supplier, 1234 AS Article, 6 AS SalesQty, 9 AS StockQty UNION
SELECT 201902 AS Month, 345 AS Branch, 1 AS Supplier, 1234 AS Article, 4 AS SalesQty, 13 AS StockQty UNION

SELECT 201811 AS Month, 345 AS Branch, 1 AS Supplier, 4321 AS Article, 3 AS SalesQty, 11 AS StockQty UNION
SELECT 201812 AS Month, 345 AS Branch, 1 AS Supplier, 4321 AS Article, 4 AS SalesQty, 5 AS StockQty UNION
SELECT 201901 AS Month, 345 AS Branch, 1 AS Supplier, 4321 AS Article, 2 AS SalesQty, 8 AS StockQty UNION
SELECT 201902 AS Month, 345 AS Branch, 1 AS Supplier, 4321 AS Article, 1 AS SalesQty, 10 AS StockQty UNION

SELECT 201811 AS Month, 333 AS Branch, 2 AS Supplier, 3122 AS Article, 4 AS SalesQty, 11 AS StockQty UNION
SELECT 201812 AS Month, 333 AS Branch, 2 AS Supplier, 3122 AS Article, 2 AS SalesQty, 4 AS StockQty UNION
SELECT 201901 AS Month, 333 AS Branch, 2 AS Supplier, 3122 AS Article, 1 AS SalesQty, 8 AS StockQty UNION
SELECT 201902 AS Month, 333 AS Branch, 2 AS Supplier, 3122 AS Article, 7 AS SalesQty, NULL AS StockQty  
)

SELECT		Branch 
			, Supplier
			, Article
			, SumSalesQty = SUM(SalesQty)
			-- My try
			/*, LatestStockQty =	(SELECT StockQty FROM TestTable i WHERE		MAX(Month) = Month
																			AND TT.Branch = i. Branch
																			AND TT.Supplier = i.Branch
																			AND TT.Article = i.Branch)*/
FROM		TestTable TT
GROUP BY	Branch 
			, Supplier
			, Article

Branch	Supplier	Article	SumSalesQty
333	2	3122	14
345	1	1234	15
345	1	4321	10