SQL don't display ORDER BY column
The variant with direct separate SELECTs:

WITH
cte21 AS (SELECT SUM(try_convert(numeric(38, 5), Rüsten_Ist)) AS Rüsten, 1 AS filtering
          FROM     [Test].[dbo].[ZLA01_Lang_DETAIL]
          WHERE  (ArbPl_Ist = 103100) AND (YEAR(BuchDatum) = 2019) AND (DATEPART(week, BuchDatum) = 21)
          UNION
          SELECT SUM(ISNULL(try_convert(numeric(38, 5), Arb__Plan), 0) + ISNULL(try_convert(numeric(38, 5), Masch__Ist), 0) + ISNULL(try_convert(numeric(38, 5), IstArb), 0)) AS B, 2 AS filtering
          FROM     [Test].[dbo].[ZLA01_Lang_DETAIL]
          WHERE  (ArbPl_Ist = 103100) AND (YEAR(BuchDatum) = 2019) AND (DATEPART(week, BuchDatum) = 21)
          UNION
          SELECT SUM(try_convert(numeric(38, 5), Ausschuß)) AS C, 4 AS filtering
          FROM     [Test].[dbo].[ZLA01_Lang_DETAIL]
          WHERE  (ArbPl_Ist = 103100) AND (YEAR(BuchDatum) = 2019) AND (DATEPART(week, 
          BuchDatum) = 21)
         ),
cte22 AS (SELECT SUM(try_convert(numeric(38, 5), Rüsten_Ist)) AS Rüsten, 1 AS filtering
          FROM     [Test].[dbo].[ZLA01_Lang_DETAIL]
          WHERE  (ArbPl_Ist = 103100) AND (YEAR(BuchDatum) = 2019) AND (DATEPART(week, BuchDatum) = 22)
          UNION
          SELECT SUM(ISNULL(try_convert(numeric(38, 5), Arb__Plan), 0) + ISNULL(try_convert(numeric(38, 5), Masch__Ist), 0) + ISNULL(try_convert(numeric(38, 5), IstArb), 0)) AS B, 2 AS filtering
          FROM     [Test].[dbo].[ZLA01_Lang_DETAIL]
          WHERE  (ArbPl_Ist = 103100) AND (YEAR(BuchDatum) = 2019) AND (DATEPART(week, BuchDatum) = 22)
          UNION
          SELECT SUM(try_convert(numeric(38, 5), Ausschuß)) AS C, 4 AS filtering
          FROM     [Test].[dbo].[ZLA01_Lang_DETAIL]
          WHERE  (ArbPl_Ist = 103100) AND (YEAR(BuchDatum) = 2019) AND (DATEPART(week, 
          BuchDatum) = 22)
         )
INSERT INTO [Test].[dbo].[Übersicht_Woche_RAP] (W21, W22, id)
SELECT cte21.Rüsten, cte22.Rüsten, cte21.filtering
FROM cte21
JOIN cte22 ON cte21.filtering = cte22.filtering


But you can create more compact version by create cte AS (SELECT 21 weeknumber UNION SELECT 22) and cross apply it to formalized version of your query (add GROUP BY and wrap values with MAX(), or use PIVOT).