When I slide the crossfader, the sound doesn’t change. Even if I slide the crossfader all the way to the left/right, I can hear the sound from the other deck.

Crossfader Assign may not be set the way you want the crossfader to work.

Select a setting other than THRU for the CROSSFADER CURVE switch.

For details, see the Operating Instructions > [Audio output] > [Channel section/master section].

You can read the Operating Instructions [here].