rekordbox one or more files were not exported. check the log file saved in the documents folder

Review the contents of the log. That will tell you why the file did not export and from there you can identify what you need to do to export the relevant tracks.

Open Terminal then type:
sudo xattr -cr (And here drag your music folder location), example:

sudo xattr -cr /library/music/techhouse

Then enter your password and try to export again! It worked for me and saved me for a gig tomorrow! 

Credits:
https://www.youtube.com/watch?v=2YQz4c7_Zf0