If JSWolf is right and this is an epub3, then Sigil is not yet ready for it unless you are doing only very vanilla editing, using none of the built in tools, etc. This will change greatly in Sigil 0.9.3, the next release (current Sigil master right now) which has much better epub3 support.
The entire Sigil 0.9.X series is meant to gradually add support for epub3 in stages while keeping full epub2 support. We are still early in that process, sorry about that.
Please try again when Sigil 0.9.3 is released.
KevinH
Last edited by KevinH; 01-11-2016 at 07:00 PM.
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