Running into this same issue...
Rekordbox Unable To Export Tracks In Library Claiming It Can't Find Them
Since updating to Mac OS Ventura 13.3.1 Rekordbox has started saying it can't export certain songs from my library, stating in the export logs that these files "cannot be found" however they are all in my Rekordbox library, analyzed and playable. I've also had them on the USB in the past so it's not an issue with the file formats.
I'm currently using Rekordbox 6.7.1
Any information on this would be well useful as I quite like being able to take music with me to my gigs.
Are those Rekordbox collections sync'd using CloudSync and Dropbox?
I'm doing a lot of fixing for artists with similar problems, the root of the problem is always CloudSync/Dropbox integration.
Can you please file an inquiry ticket here and our support team will gather some additional information to assist you in resolving this issue.