[Iznogood]

This means that in your opf-file, there is an entry for a file called CoverImage.xhtml, but the file cannot be reached (i.e. it does not exist). Correct the opf file and you're good to go

Edit: I see that the screenshot is different from the text quoted in your post. The screenshot indicates that the file CoverImage.xhtml is used as a reference somewhere, e.g. in a link. You could search for CoverImage in your book to find out where it's referenced, probably in one of the xhtml-files or the ncx file