Prior to WordPress 2.9 there wasn’t featured image (post thumbnails) support in WordPress so many themes used meta options or custom fields for defining featured images or they didn’t have any sort of default post image.
So maybe you are switching from an older theme to a new one that does support featured images or maybe your prior theme was just very minimal and didn’t use them. We’ll you aren’t going to take hours to go through your hundreds of posts to set the featured image (or maybe you are) so there is a quick solution for this and it comes in the form of a plugin!
No Featured Images Error Solution
A simple solution is to use the “Easy Add Thumbnail” plugin to automatically set featured images on old posts. Just install, activate and follow the instructions to add images to old posts. Going forward, just remember to add your featured images when you publish new posts.