Thanks for posting in our Community about this issue.
In order to investigate this, we’d like you to test if the DDJ-400 is being show in the Audio MIDI Setup app. Follow these steps:
(1) Connect DDJ-400 to your computer via USB cable.
(2) Open [Applications] folder, [Utilities] folder and [Audio MIDI Setup.app].
(3) Open [Audio Devices] window. Confirm that [Audio MIDI Setup] is displayed on Menu bar.
Click [Window] and choose [Show MIDI Studio] or [Show MIDI Window]. (The displayed name depends on OS version.)
(4)[MIDI Studio] window opens up, click [DDJ-400] icon. The icon turns to blue color, click [Remove Device] or [-] icon at upper of the screen.
“DDJ-400” icon disappears and comes back again.
(5) Close [Audio MIDI Setup] and remove the USB cable from DDJ-400.
(6) Connect DDJ-400 to your computer via the USB cable and run DJ Application.
Let me know if the issue persists.