eBay displays this notice in a auction page, when it believes it is not possible to download product pictures from your website link that had been generated by the module.

Note that Fastbay only sends the link of the image on your website in the selected image format to eBay, which will download it in the hours following publication
to make it available in its internal “hosting”.

Generally, if the photo is present and visible in the gallery of the listing, it could be a temporary warning that you can ignore,
otherwise, make sure that your site / server is reachable and there are no network problems or blocks (firewall / ip ban) that could prevent eBay from retrieving the images from Prestashop.