I'm confused. Your original post says it's asking for your PR card but subsequently you refer to the issue being for your parents travelling to visit you in Canada? Who is trying to check in - you or them?
A non-PR, British Citizen obviously won't have a PR card but will need an ETA. So, if I follow you, a UK PP holder is being directed to a check in field to enter PR number? That all sounds a bit random and obviously wrong. I'd certainly clear the check in session, clear cookies - perhaps use incognito mode so no details are carried forward - and try again. If it fails then suggest as others say, get your parents to the airport early to checkin manually.