It is as Alpe says. You may have eliminated it, thinking it was unessential, but there is a reference to it. It is easy to do if you have eliminated a lot of extra page numbers and so on to create an epub, but there were references to things you eliminated. You can simply search for the reference in the TOC and eliminate it there, and it won't be an error any more. But it won't be a link anymore either, if you wanted one. You will have to put in the link again yourself.
Sigil works very well for this kind of cleanup and it is free and getting better and better all the time.
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