Error "Could not download file"
When you optimize an image with ShortPixel Image Optimizer, you may receive the message "Could not download file".
The most common cause of this error is that the image and/or its thumbnails are listed in the Media Library (and therefore exist in the database), but no longer exist on disk physically. To check this, download your Optimization Report from this page and search for the image in question. Once you find it, you will also see the thumbnails. For example:
Now copy the URL of each image or thumbnail and open it in an Incognito/Private browser window until you figure out which is the problematic image that will not open.
For more causes and troubleshooting methods, please read this article: Why some or all of the images cannot be optimized?