The HTML spec itself does also say that the link element is allowed in the body if it has an itemprop attributebut only if the link element doesn't have a rel value. (itemprop is Microdata attribute whose purpose is basically the same as the RDFa property attribute).
And while the HTML spec itself says link is normally not allowed in the body, the RDFa spec says that if a link element has a property attribute, it is allowed in the body.
So that validator message is basically saying,The link element is only allowed here if it has a property attribute. But this particular link element doesn't have a property attribute.
So we have two different attributes that both independently affect where in a document the link element is allowed to appear, and that complicates the checking logic in the validator in a way that makes it difficult to emit a better, more helpful error message for this case.
W3C HTML5 validator maintainer here. As pointed out in another answer, in addition to checking requirements in the HTML5 spec itself, the validator also checks against requirements in the HTML+RDFa 1.1 spec: