Important notice for Cloud Library Sync users.
In rekordbox ver. 6.0.0 and 6.0.1, the error message "You cannot import tracks that have been already imported and uploaded to the cloud." may appear, meaning you'll be unable to play or export tracks that are uploaded to the cloud using Cloud Library Sync from iTunes/Explorer/Inflyte in the Tree View.
[6/24 added] This problem has been fixed in ver. 6.0.2. Please use the latest version of rekordbox .