title_body: container.xml could not be found
upvoted_answer: This sounds like a minor error. I have a META-INF/container.xml which I have never looked at until now. The entire file says: 
  
  &lt;?xml version="1.0" encoding="utf-8" standalone="no"?&gt;
    &lt;container xmlns="urn:oasis:names:tc:opendocument:xmlns:container" 
     version="1.0"&gt;
       &lt;rootfiles&gt;
          &lt;rootfile full-path="OEBPS/package.opf" media-
           type="application/oebps-package+xml"/&gt;
       &lt;/rootfiles&gt;
   &lt;/container&gt;
  
  
  The specification for this file is here: http://www.idpf.org/epub/31/spec/epub-ocf.html#sec-container-metainf-container.xml
  
  By the way, if you don't already do this, you should always use the epub validator to validate your epub file. (This error message sounds like the output of an epub validator). 
  
  (Let me add that my ebook production process generates the container.xml file automatically. Sigil should do the same. Perhaps you changed a setting or accidentally deleted this file?)