I updated to LR6.4 today and since have been unable to import any photos. The following message always comes up:- Some import operations were not performed. The following files were not imported because they could not be read. I did not change anything, only updated. I have since rebooted my Mac, uninstalled Lightroom and reinstalled LR6 but all to no avail. Very frustrated!
Seems like the destination location is not set correctly.
Make sure that the destination location in the import window is set correctly and has full read and write permissions.