It sounds like LR is wanting to write some temporary files to do system-defined $TMPDIR or $TMPD and cannot due to either a permissions problem or folders that don’t exist, anymore, and LR cannot recreate them.
Here is a similar discussion, rather long, about the same issue, where it doesn’t end with the user saying things are ok, but maybe what the help said did eventually work:
Sounds like your permissions on your boot drive are messed up and Lightroom is not able to recreate or write in some folders it needs. It is also possible that your account is no longer an administrative account. Check in System Preferences->Users & Groups whether your log on account is still Admin. Lightroom doesn't need to be run as an admin account but it does need admin privileges if it needs to recreate directories that were deleted. Then run Disk Utility (In /Applications/Utilities) and do a Repair Disk Permissions on your boot drive. Lightroom should be able to recreate the folders by itself. If this doesn't work, reinstall Lightroom. It is possible that the "geniuses" deleted some essential directories that you can only get back by reinstalling.