Answer: SQL Error with Order By in Subquery

If you're working with SQL Server 2012 or later, this is now easy to fix. Add an offset 0 rows:

SELECT (
  SELECT
  COUNT(1) FROM Seanslar WHERE MONTH(tarihi) = 4
  GROUP BY refKlinik_id
  ORDER BY refKlinik_id OFFSET 0 ROWS
) as dorduncuay