File Not Found after export

"File not Found" means that LrC cannot find the image at the location that it expects, i.e. what was recorded in the catalog when the image was first imported. You see the image in the Library module, because there you are only seeing a preview of the original image.....check again in the Library module and there's likely an exclamation mark in the top right corner of the grid thumbnail (which means it's missing).

There are many reasons why an image could be missing, but you have to try to figure out what's happened. Start with this document: https://www.lightroomqueen.com/lightroom-photos-missing-fix/

Export has nothing to do with file location and doesn't change it. If you're exporting, then reimporting at the the same time, and then hoping to edit the export, it's probably a bad idea as it degrades the image (assming a JPEG). 

Go back to the file in Library. There'll be an exclamation mark on it. Click this to open the expected location. Is the file there? If not search for the file name in the OS and then point at it from the Lightroom dialog.