Would you please follow the steps below if you cannot access Serato DJ on El Capitan with DDJ-S9?
1) Uninstall the DDJ-S9 driver currently installed.
・Please uninstall the driver by using the uninstaller included in the driver for El Capitan Ver.1.1.1 downloaded from the Pioneer DJ site below.
・If you may have also installed S9 driver for Yosemite Ver.1.0.1, uninstall it using the uninstaller included in the driver for Yosemite Ver.1.0.1 downloaded from the same Pioneer DJ site.
2) Install the driver included in Serato DJ 1.8.1.
After finishing 1), if you connect DJM-S9 and Serato DJ 1.8.1, the message to ask you to install the driver.
Please install the driver by following the instructions shown in the screen.
3) Please check if the same issue does not occur.
If you cannot solve the issue, please let us know.
Thank you very much for your kind cooperation in advance.
CRM, Pioneer DJ