@C-novice...Since you are a novice with Sigil(and html coding) the best I can suggest is that you start from scratch again using your LibreOffice doc. This method to obtain an epub 3 version of your ebook wont take long, only involves a few steps and does not involve any coding in HTML.
Here's my suggestion:
* Download and install the
HTML2Epub Sigil plugin from
here. Read the release notes for this plugin!!
* Download and install the
ePub3-itizer Sigil plugin from
here. Read the release notes for this plugin!!
* If you haven't already done so, download and install the
Epubcheck Sigil plugin from
here.
* Export your doc from LibreOffce as HTML(Writer) + images into a new folder on your desktop. See the
HTML2Epub release notes for more details.
* In Sigil go to
Plugins > Input and click on
HTML2Epub. This plugin will automatically convert your html doc to full epub 2 format and import it into Sigil.
* After running the plugin, check the epub in Sigil using the
FlightCrew or
Epubcheck plugin to make sure that you have a valid epub 2 file.
* If everything is OK, then select
Plugins > Output > ePub3-itizer and run it. This plugin will convert a valid epub 2 file to valid epub 3 format and save it to your desktop.
* After conversion run
Epubcheck on your new epub 3 file to be sure.
Also be aware that the
HTML2Epub plugin will remove any blank lines caused by the enter key. So make sure that your headings, text and all spacing(space before, space after etc) in your LO doc have been properly styled using paragraph styles only. Unfortunately, since you cannot code in html, if you spot any formatting errors in your epub 3 file, however trivial, then you will have to go right back to your LO doc to correct it and then run the above process all over again.
I'm also guessing that you probably used the eLAIX extension for LibreOffice to convert your LO doc to epub 3. I tried using it once -- very complicated -- I dumped it after only 5 minutes. I mean what can you say about a LibreOffice add-in that has 35 toolbar buttons? That's even more toolbar buttons than LO has on its default toolbars!!..
