Why do I get SQLCODE -206 when I run a query to calculate median value of a column?

I am trying to find median of the field "bal" based on "key" in DB2. I get this error when I execute this code "Error: DB2 SQL Error: SQLCODE=-206, SQLSTATE=42703, SQLERRMC=KEY, DRIVER=3.66.46, SQLState: 42703).

Appreciate your comments !!

        SELECT
           key,
           AVG(bal)
        FROM
        (
           SELECT
              key,
              bal,
              ROW_NUMBER() OVER (
                 PARTITION BY key 
                 ORDER BY bal ASC) AS RowAsc,
              ROW_NUMBER() OVER (
                 PARTITION BY key 
                 ORDER BY bal DESC) AS RowDesc
           FROM tab1
        ) x
        WHERE 
           RowAsc IN (RowDesc, RowDesc - 1, RowDesc + 1)
        GROUP BY key
        ORDER BY key;

Ensure that every reference to key becomes "key"

SELECT
      "key"
    , AVG(bal)
FROM (
      SELECT
            "key"
          , bal
          , ROW_NUMBER() OVER (PARTITION BY "key"
                               ORDER BY bal ASC)  AS rowasc
          , ROW_NUMBER() OVER (PARTITION BY "key"
                               ORDER BY bal DESC) AS rowdesc
      FROM tab1
) x
WHERE RowAsc IN (RowDesc, RowDesc - 1, RowDesc + 1)
GROUP BY
      "key"
ORDER BY
      "key"
;