Track titles are grayed out on my computer and I can’t play them.

When a track title is grayed out, it means the file hasn't been transferred from your other computers/mobile devices. To play the track in rekordbox, you need to transfer it from your other computers/mobile devices in one of the following ways:

File transfer using Mobile Library Sync

Connect rekordbox (Mac/Windows) and rekordbox (iOS) to the same Wi-Fi network.

In rekordbox (iOS), tap [Settings] > [Connect to rekordbox (Mac/Win)].

In rekordbox (Mac/Windows), click [SEARCH MOBILE].

Select the track from a track list in rekordbox (Mac/Windows) and right-click to show the context menu. Select [Mobile Library Sync] > [Transfer to this computer].

Using Cloud Library Sync

Open the information window and check which device contains the track with the grayed-out title.

Upload the track from the device that contains it.

rekordbox (Mac/Windows): Select the track from a track list, right-click and select [Cloud Library Sync] > [Upload] from the context menu.

rekordbox (iOS): Swipe a track to the left, tap [...], and select [Upload to Cloud].

3. Once the track has been uploaded, it will be downloaded automatically by the Dropbox desktop application installed on your computer.