Hi patriciac ,
Please make sure you are using Lightroom 5.7.1.Check from Help>>Updates.
Have you used a Preset before exporting the image?
When I go to Updates, the version shows 5.7 and when I run the check for updates, the message
shows "Your software is up to date. No updates are available at this time."
I have used a Preset to export. I have tried recreating the export and the result is the same: my older images from several years ago receive the export error, while the newer images (from the collection) export without error. The entire collection exported as a whole, receives
the export error: "An internal error has occurred: ?:0: attempt to index a nil value." But, when I individually export newer images, they will export successfully.
I tried exporting the collection to a new catalog. But the same error message appeared.
Many thanks for your quick response. This will be a major issue if unsolved.