Thank you for contacting us for help. I know that error does not say why the photo does not export, so it is good to try a few simple things to hopefully solve the problem or at least understand the problem better.
Could you try restarting your device, and quitting other apps, and then try Export again in Lightroom? I wonder if there's a problem with reading the image from the file system or with device memory that restarting might somehow help.
If you sign into your same account at http://lightroom.adobe.com are you able to access and export your images as full size copies? And as Originals? (I just want to make sure a healthy copy of your image is safely stored in Lightroom's online cloud storage.)