For those who have been unable to link their PR card application: in my case, I had to use the birth city / town (with exact punctuation, including state / province) corresponding to *an older passport*. I had to renew my passport a couple years ago; evidently, even though I updated CIC about the passport change, I still had to use the birth city as per the old passport. The older one used only the initials of the birth state, unlike my "new" passport, which spells out the state entirely.
For completeness:
COPR: Apr. 01, 2021
Sent photos by post: Apr 09, 2021 (no tracking, so I don't know the receive date)
No updates since, because I was unable to link the application until just now. But at least now I know my photos weren't stolen by the carrier pigeon.