Official comment
Okay, so here's the deal... if you're using SYNC MANAGER, you need to go to the top-right and select the DEVICE LIBRARY PLUS from the drop-down, even if you have already checked it in the rekordbox preferences. This is a hiccup that could not be resolved before launch, but the next version will have it wrapped up.
If you are exporting directly by drag-and-drop, or right-click > export, you will have no problems.