SQL Error with Order By in Subquery

This is the error you get (emphasis mine):

The ORDER BY clause is invalid in views, inline functions, derived tables, subqueries, and common table expressions, unless TOP or FOR XML is also specified.

So, how can you avoid the error? By specifying TOP, would be one possibility, I guess.

SELECT (
  SELECT TOP 100 PERCENT
  COUNT(1) FROM Seanslar WHERE MONTH(tarihi) = 4
  GROUP BY refKlinik_id
  ORDER BY refKlinik_id
) as dorduncuay

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