"union" dan önceki ";" kaldırıp öyle dener misiniz?
SELECT SSD1.SAHA_MUSTERI, SSD1.SAHA_URUN_UST_ADI, SSD1.SAHA_URUN_KOD, SSD1.SAHA_URUN, SSD1.SAHA_RENK, SSD1.Kalan, SSD1.NET_REZERV
FROM SSD1 INNER JOIN MSR2 ON SSD1.SAHA_MUSTERI = MSR2.SAHA_MUSTERI
WHERE (((SSD1.Kalan)>=0))
ORDER BY SSD1.SAHA_URUN_KOD
UNION SELECT MSR2.SAHA_MUSTERI, MSR2.SAHA_URUN_UST_ADI, MSR2.SAHA_URUN_KOD, MSR2.SAHA_URUN, MSR2.SAHA_RENK, MSR2.Kalan, MSR2.NET_REZERV
FROM SSD1 INNER JOIN MSR2 ON SSD1.SAHA_MUSTERI = MSR2.SAHA_MUSTERI;