I cannot record in rekordbox.

Please check the following.
(1) Is “USB REC” displayed in the recording source selection window of the rekordbox REC panel?
If not displayed: The computer has not recognized the device. Check if the USB cable is connected correctly to the computer.
(2) When playing a record, does the level meter on the rekordbox REC panel move?
If the level meter does not operate, or if the meter does not move: The recording level may be set low. Use the recording adjustment knob to set an appropriate recording level.