Msg 8114, Level 16, State 5, Line 1 Error converting data type nvarchar to float
Tha sollution is (Thanks @Akina) To use the CAST() functino and convert all the select statements to datatype CHAR.

Sample code:

SELECT CAST(SUM(try_convert(numeric(38, 2), Rüsten_Ist)) AS CHAR) AS Rüsten, 1 AS filtering
          FROM     [Test].[dbo].[ZLA01_Lang_DETAIL]
          WHERE  (ArbPl_Ist = 103100) AND (YEAR(BuchDatum) = 2019) AND (DATEPART(week, BuchDatum) = DATEPART(week,GETDATE())-8)