PIVOT with MONTH()

If you move the Month function into a prior sourcetable the PIVOT works. Note I don't think you need to go grouping things following the pivot.

SELECT
    def_kstnr,
    [1] AS Jan,
    [2] AS Feb,
    [3] AS Mrz,
    [4] AS Apr,
    [5] AS Mai,
    [6] AS Jun,
    [7] AS Jul,
    [8] AS Aug,
    [9] AS Sep,
    [10] AS Okt,
    [11] AS Nov,
    [12] AS Dez
FROM
(Select 
def_kstnr,
def_zeit,
 MONTH(def_datum) as TMonth
  from
    dbo.def) source
PIVOT
(
    SUM(def_zeit)
    FOR TMonth
    IN ( [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] )
) AS pvtMonth

--Create Temporary Sales Table
CREATE TABLE #Sales
(SalesId INT IDENTITY(1,1), SalesDate DateTime)
GO
--Populate 1000 Sample Sales Records With 
--Random past 0-798 days as sales date
INSERT INTO #Sales(SalesDate)
VALUES(DATEADD(dd, - CONVERT(INT, (798+1)*RAND()),GETDATE()))
GO 1000

Demo 1: Getting Monthly Data

SELECT YEAR(SalesDate) [Year], MONTH(SalesDate) [Month], 
 DATENAME(MONTH,SalesDate) [Month Name], COUNT(1) [Sales Count]
FROM #Sales
GROUP BY YEAR(SalesDate), MONTH(SalesDate), 
 DATENAME(MONTH, SalesDate)
ORDER BY 1,2

RESULT:-

Demo 2: Getting Monthly Data using PIVOT

SELECT *
FROM (SELECT YEAR(SalesDate) [Year], 
       DATENAME(MONTH, SalesDate) [Month], 
       COUNT(1) [Sales Count]
      FROM #Sales
      GROUP BY YEAR(SalesDate), 
      DATENAME(MONTH, SalesDate)) AS MontlySalesData
PIVOT( SUM([Sales Count])   
    FOR Month IN ([January],[February],[March],[April],[May],
    [June],[July],[August],[September],[October],[November],
    [December])) AS MNamePivot

RESULT:-