I am getting error in the following Query [closed]

SELECT category 
FROM
    (SELECT  Category
     FROM Table
     ORDER BY 
         CASE
             WHEN Category = 'A' THEN 1
             WHEN Category = 'C' THEN 2
             WHEN Category = 'D' THEN 3
             WHEN Category = 'e' THEN 4) a
ORDER BY 
    Category