You are trying to make an ePub 3 eBook in a program that makes ePub 2 eBooks.
Do you really need to make ePub 3? Are you using any ePub 3 specific features? can what you want to do be done in ePub 2? If you do make a specific ePub 3, not many people will be able to read it. So if you aren't doing anything ePub 3 specific then just go ePub 2.
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