MySQL menggunakan Sum dan Case

Kesalahan ini disebabkan oleh spasi antara nama fungsi dan tanda kurung

SUM (CASE WHEN ...
   ^^

Baca selengkapnya Penguraian dan Resolusi Nama Fungsi

Coba

SELECT BusinessUnit,
       SUM(CASE WHEN OrderDate = CURDATE() THEN 1 ELSE 0 END) TodaysOrders,
       SUM(CASE WHEN DATE_FORMAT(OrderDate, '%Y%m') = DATE_FORMAT(CURDATE(), '%Y%m') THEN 1 ELSE 0 END) ThisMonthsOrders,
       SUM(CASE WHEN YEAR(OrderDate) = YEAR(CURDATE()) THEN 1 ELSE 0 END) ThisYearsOrders
  FROM OrderTable
 WHERE Canceled <> 'Y'
 GROUP BY BusinessUnit

Ini SQLFiddle demo