ORA-00923: FROM keyword not found where expected - LISTAGG

Question

I have to comma separate all the values of a column and for this i am using LISTAGG function, but getting error

"ORA-00923: FROM keyword not found where expected"

Please find below code i am using. Kindly suggest why i am getting this error.

select listagg(t.user_email, ', ') WITHIN GROUP (ORDER BY t.user_email) AS DelimitedEmail
FROM (
SELECT  user_email 
  , sum(LENGTH(user_email)) OVER (ORDER BY user_email) R1
  , row_number() over (order by user_email) rn
from tbl_Users
) t
WHERE t.R1 <= 4000;