[Adoby]

I don't really think that calibre is intended as a tool to use to create ebooks. Even though it can be used for that. Calibre strengths are to manage a collection of books, handle devices and convert between formats.

If you intend to publish or spread your book some other way, as I assume is the case, I strongly recommend that you learn and use Sigil instead. If you try to use calibre I suspect that you will encounter a lot of problems, or your readers might.

I really don't know if it is supposed to be possible to do what you tried to do with calibre, but even if it is, I would recommend that you use Sigil instead. It is the right tool.

Sigil is nice and easy to use. But very powerful as well. And the strength of Sigil is actually to create ebooks.

The best way to learn is most likely to just do it. Start with a small test book. Just a few pages. Learn how to export html from Libre Office and import in Sigil. How to embed fonts. How to adjust the layout. How to create table of contents. How to verify that the ebook is correct. And so on.

And if you get stuck there is a subforum for Sigil here...