Please check all epub3 books with EpubCheck before running this plugin, because it might fail if the input file is invalid.
The plugin failed because there's no time stamp entry in the .opf file. (Time stamps are mandatory in epub3 files.)
Time stamps look like this:
Code:
<meta property="dcterms:modified">2019-12-01T10:06:19Z</meta>
To fix this issue, add and delete a space in an HTML file and save the epub with Sigil. This'll add the missing time stamp.