Files can't be written. Most likely the folder you are trying to write the exported files into does not have WRITE permission. So go into your operating system and change the permissions; or try another folder where you already have WRITE permission.
By the way, next time please provide a meaningful title to your post.
Please provide additional information such as OS and whether you are trying to export to the same drive as where your images are located or a different drive. As dj_paige mentioned, this is possibly a permissions issue.