The image_link or additional_image_link URL is broken, redirected, or does not end in .jpeg or .png. TikTok cannot fetch the image so the product is marked unavailable.
TikTok Shop / TikTok Catalog Manager requires product images that are publicly reachable, sized within the platform's bounds, in supported formats (JPEG/PNG typically), and free of overlays, watermarks, retailer logos, or text callouts. This issue trips when one of those constraints is broken.