[KevinH]

As Jellby stated, your package tag has no namespace defined for it.

Just add:

xmlns="http://www.idpf.org/2007/opf"

to the other namespaces.

That line says that things in the opf namespace do NOT need a prefix.

Or do as Jellby suggested and use the opf prefix you did define opf: on both the package and metadata tags manifest, spine, etc, etc.

I typically use:

xmlns="http://www.idpf.org/2007/opf"

on the package tag to say no prefix means that tag is in the opf name space,

but then use the remaining namespaces including the opf:, dc:, dcterms:, and tei: terms *only* added to the metadata tag for its children to use.

You end up with a much simpler and cleaner opf.