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Actually, you need not create an id from a file name. Sigil does it that way because if filenames are unique that the id will be unique.
So IF all links are truly valid, then feiyr.com's validator is broken for epub2. Either feiyr.com wants an epub3 instead of an epub2, or using the id "cover-image" which incidentally matches an official epub3 manifest property name "cover-image") is somehow confusing their validator.
This all assumes ALL your opf href entries are valid! If not, just fixing them should help.
Try as Turtle suggested, changing the metadata content to be match its filename or any other unique id that you set as the id in the cover.jpg manifest entry. Anything but "cover-image".
If that helps, then the bug is in there validator. If not, then check if feiyr.com only accepts epub3.
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