“Image not found in the Media Library”

Alt fixes resolve each on-page image to a Media Library item by filename. If the image isn't in the Library, it's hotlinked from another domain, embedded by a theme/plugin, or served from a CDN with a different filename, there's nothing to write to.

• Images uploaded through the WordPress Media Library will resolve and get fixed.
• Theme/builder decorative images and externally-hosted images can't be set via the Library.
• For those, add alt text where the image is inserted (the builder module or the external source).