Sorry, no files are available or folder '/en/media/photos/graduation38_39/cos/thumbs' may not exist.
You have to select the correct folder which contains the thumbnail files from production.
Also, you have to make sure that all thumnnail files are having the extension .jpg
or .png and the file size of each thumbnail is no more than 10240 bytes.
For more details, please check the following site How to use Gallery?.