The aliases will given after generating result set so you can't use them in Windowing functions. You can do this:
SELECT * FROM
(
SELECT sym.*,
(
CASE WHEN shr_last_updt_dt >= trd_last_updt_dt
THEN shr_last_updt_dt
ELSE trd_last_updt_dt END
) AS LatestUpdatedDate,
row_number() over ( ORDER BY
(
CASE WHEN shr_last_updt_dt >= trd_last_updt_dt
THEN shr_last_updt_dt
ELSE trd_last_updt_dt END
) DESC ) AS line_number
FROM trdg_sym sym
WHERE ((wkly_rpt_dt = :settleDate)) AND ((lower(rpt_type_cd)=lower(:reportType)))
)
WHERE line_number BETWEEN 1 AND 25 ORDER BY line_number
or wrap your subquery to another
SELECT * FROM
(
SELECT *,
row_number() over ( ORDER BY LatestUpdatedDate DESC ) AS line_number
FROM
(
SELECT sym.*,
(
CASE WHEN shr_last_updt_dt >= trd_last_updt_dt
THEN shr_last_updt_dt
ELSE trd_last_updt_dt END
) AS LatestUpdatedDate
FROM trdg_sym sym
WHERE ((wkly_rpt_dt = :settleDate)) AND ((lower(rpt_type_cd)=lower(:reportType)))
)
)
WHERE line_number BETWEEN 1 AND 25 ORDER BY line_number
P.S. I don't sure for oacle
syntax, but should you give aliases to subquery?