A file cannot be played.

Is the file copyright-protected (by DRM)? Copyright-protected files cannot be played.
Furthermore, the file may be damaged or it may have been renamed or moved. Confirm the status by checking the library status icon.
For details, see "Managing music files" in the rekordbox software manual. The rekordbox software manual can be obtained from the rekordbox website.
https://rekordbox.com/en/support/manual.php