This is a Walmart specific error.
Your Walmart listing does not have any images, and you need to add them.
Your Walmart listing has images, yet this error message is still displayed. There is no reason for this message to be displayed, but LM cannot get around the error if that is what Walmart is sending us. It is not a real error message, just an oddity on Walmart's side.
If your Walmart listing lacks images, add those images and the error should be resolved.
If your Walmart listing already has images, then you will need to re-sync the listing to Walmart to get the error message to go away. This may need to be done several times. You can do so, by clicking Save Changes on the Edit Listings page for the Walmart listing. See image.