When left on its own, Google's canonicalization is generally done at the URL level, not on the domain. So it's not unusual for sites to have pages show up for variations of their domain name/URL and sometimes run into duplicate content issues. But, as I say, when a URL *has* been canonicalized by Google, it's no longer indexed, in which case, Google's tool won't find the underlying data required for author verification for that URL. You can only verify the canonical. And this all assumes that Google has ever seen the non-canonical version of the URL. Naturally, you'll also be unable to verify a URL that's never been crawled.