This thread has some useful info:Hello everyone.
How to link up the pr card application online?
https://www.canadavisa.com/canada-immigration-discussion-board/threads/linking-pr-card-application-to-track-pr-card-after-landing-flagpole-landing-interview-is-complete.617040/
Try all possible combinations of adding the PR application, the Place of Birth option didnt work for me. But some other combination worked, and I was able to link my PR card application online.