An easier way to edit your ebook details or metadata is to download The Epub Metadata Editor -- which is a free download from GitHub. This is a simple editor for epub metadata which is very easy to use. After you have entered and saved all the data you can then check the entries by double clicking on the content.opf file in the Sigil Book Browser and inspect all the <metadata> entries.
Why not try my free Sigil plugins now available on MobileRead?
OpenDocHTMLImport - converts ODT HTML(Writer) docs directly to epub.
GoogleZipImport - converts Google Doc HTML directly to epub format.
NormalizeScrivEpub - reformats, finishes-off and normalizes Scrivener epubs.
Emma, I know you have this problem, but really, I don't experience it at all. You must CLICK on the "metadata" option -- Title, Author, whatever -- and it turns dark blue. Now you can type into the window.
If this genuinely doesn't happen, go to MobileRead dot com and skip down to the Sigil forum (it's under Ebook Software, something like that, the third major subject area). The two boffins who maintain this wonderful software are active members, and there are other very knowledgeable people there. (They can also be very intolerant, so prepare yourself to ignore snark.) Ask for help, and you will get it, I promise. Sigil is a wonderful software and it will reward you for the care you put into it at the beginning. I've used it for six years and I'm still discovering new and more powerful tools.
(Don't trust KDP to publish a print edition. Don't trust CreateSpace to publish an ebook.)