In future, you'll get better response (from me, at least) posting in the thread for the plugin rather than making a new one. In this case, the
EpubSplit Thread.
Your error is caused by the ebook TOC metadata missing a tag that EpubSplit expects to find. Generally, that's because the ebook metadata is flawed.
Editing and saving the epub in calibre's book editor or doing a convert from epub->epub will fix many metadata problems. Try one or both of those and then try splitting again.
FYI, EpubSplit can't work on DRM encrypted epubs.