getting error in sql

HI I am getting error: Keyword ROW_NUMBER not expected. Valid tokens: FROM
INTO.

what i am try to do is, orders have sales codes in the table clspaytp.
sales codes means like web chat, phone call, etc. where the rep is involved.
all orders get the 001 code automatic. but sometimes the reps enter more
than 1 code per order besides the 001. what I want to select is, only the
001 and the first code then that was entered (ottrnd, ottrt is date and
time). I probably dont need the case 001.


Select * from
(
    Select *
     row_number() over (partition by [Otord#]
                       order by case ottrnc when '001' then 1
else 2 end
                       , ottrnd, ottrt
                       )
                    as RowN
    from Rklib.clspaytp
) a
where a.RowN in (1,2)