Why am I getting ORA-00937 error using only aggregate functions? Here is a sample of what I'm trying to do:

SELECT
  COUNT(T1.FIELD) +
  (
    SELECT
      COUNT(T2.FIELD)
    FROM
      TABLE2 T2
  )
FROM
  TABLE1 T1


I don't see a need for group by clause in here.
Aggregate functions, but not on the same object, in a subquery, with addition. Use this:

SELECT
  (SELECT COUNT(T1.FIELD) FROM T1) +
  (SELECT COUNT(T2.FIELD) FROM T2)
FROM DUAL;