This may be due to changes in MacOS X (if that is what you upgraded). Go to MacOS 'System Settings - Security & Privacy' and add Lightroom Classic to "Full Disk Access". Click on the lock icon to unlock the settings, then click on the plus icon to add Lightroom Classic if it isn't already listed. If it is listed, then just enable the checkbox.
Check in the very last section of the export dialog. Make sure it is not blank.
"This error occurs because the export file is corrupted. There can be two solutions for the error: Solution 1: 1) Check for the ‘Post-Processing’ section of your dialog 2) In the dropdown of ‘After Export’ make sure you have a valid entry selected"