@crankypants:
Your dc namespace attribute is missing the final slash:
Code:
<metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">
BTW, your modification date entry format is also invalid:
Code:
<dc:date opf:event="modification">20160619</dc:date>
You'll need to use:
Code:
<dc:date opf:event="modification">2016-06-19</dc:date>