If rekordbox is in EXPORT mode, it cannot be controlled from the unit. Click the part indicated by [EXPORT] at the top left of rekordbox screen and then select [PERFORMANCE].

If your computer is a Mac, disconnect the USB cable that is connected to the unit and then start the Audio MIDI Setup utility software of OS X and display the MIDI window or MIDI studio. Select the [DDJ-RX] icon and then click [Remove Device]. (The icon may be [USB MIDI Device] instead of [DDJ-RX]. If so, select [USB MIDI Device].) Connect the unit with the USB cable again.