Hi,
For a shipping method to appear for Canada Post on the checkout, the products need to have a weight and dimensions and you need to use a real world address as a shipping address.
Is that the case ?
Also, deactivate temporarily the other shipping methods. That way, no shipping method will be found and the system should display the error message from the Canada Post shipping method. It should allow you to understand what's missing.
For example, it's possible that the shipping plugin it not able to retrieve the rates from the Canada Post server because it can't connect to it if cURL is not available on your server, or your hosting provider doesn't allow outgoing connections.