Did you actually receive a new PR card in August? Or is that when you applied?
The email should have included more information about what "case" the appointment is in reference to.
Depending on what the appointment is in reference to and is about, it is probably a "non-routine" procedure.
There are way, way, way too many situations which may involve an appointment with IRCC to try enumerating them without knowing some particular circumstances. You must have some idea what this is about.
GUESSING . . . in part based on older posts . . .
Not sure, not at all familiar with your situation, but I have the impression you may be outside Canada, and this is about a pending PR card application, and possibly about compliance with the PR Residency Obligation. If so, and particularly if you are relying on credit toward RO compliance for time abroad while employed by a Canadian business, it will be important to attend or reschedule the appointment, and to have all the relevant information to support your claim to this credit.
Again, this is just a guess. If I am in the ballpark, you should be aware that actually qualifying for the
employed-abroad-by-a-Canadian-business RO credit is TRICKY and many, many PRs have mistakenly relied on this credit when they do not actually meet ALL the requirements, and "ALL" the requirements must be met to get the credit.
If my guess is NOT in the ballpark . . . not at all sure what this is about. As I noted, you should have some idea, probably a good idea, what it is about.
Link to webform and telephone number (1-888-242-2100), with additional related information, can be found here:
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/contact-ircc/client-support-centre.html
Of course the telephone call can ONLY be made from IN Canada.