RB has to be in export mode and you most likely need a driver if you are using a pc
in page 31 of the manual it explains it
rekordbox Link Export
You can directly select and play tracks in rekordbox with this unit when the unit is connected
via a USB cable to a PC/Mac running rekordbox. You can play tracks with cues, loops, and
Hot Cues previously set in rekordbox.
• Install the rekordbox (Mac/Windows) USB connection (Link Export) driver software before
connecting the unit to a PC/Mac with a USB cable (page 13).
• For information on the connection with rekordbox for iOS, refer to the FAQ at the URL
• You can’t use mobile devices running rekordbox for Android with the unit.