El Capitan is not supported by rekordbox.
You would also need at least 16GB RAM for track separation function.
https://rekordbox.com/en/download/#system
You can try to rename or delete following setting file.
/Users/username/Library/Application Support/Pioneer/rekordbox6/rekordbox3.settings
*User Library is a hidden folder. You can access it with [Go] menu of Finder while holding [Option] key.
So rekordbox will start up with Track Separation disabled.
Please note other settings in Preferences will also turn back to default value.