Sounds like you broke the first commandment in Lightroom: Never Delete, Add, Move or Rename a File or a Folder from outside the Lightroom interface.
To build your exported file, Lightroom needs two things, the original file and the instructions for its editing. In your case, the original files are not found where expected by Lightroom so it is unable to edit them. Did you move, delete, rename them without telling Lightroom?
To fix this, you will have to either undo what you did outside of Lightroom, or, use the find missing files procedure to locate missing files and then manually relink them.