Most likely your cover image isn't found, because you didn't include a path in the
xlink:href value.
Code:
xlink:href="cover.jpeg"
However, unless you didn't create the book with Sigil, it should have the following path, if your book has an Images folder and you added the cover image to it:
Code:
xlink:href="../Images/cover.jpeg"
You might want to install the
FlightCrew and
EpubCheck plugins and check your book with it.
You also might want to right-click the cover image file in the Book Browser and make sure that there's a check mark before the Cover Image entry.