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You are designing an empty epub. To do that you have to say where to put your OPF file and what to name it. Only 1 opf is allowed. Since you default to epub2, you also need to do the same thing for an NCX. Again only one NCX is allowed. In your case you do not have any of either required file.
The error messagesare quite clear, without an NCX and without an OPF you will not have a valid epub (empty or otherwise).
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