Hello,
I already told you that I have contacted Canada Post numerous times and I have the correct credentials needed for this plugin so it's completely useless for you to even mention contacting them again. Obviously there is a flaw in your extension that you may want to consider looking at seeing as how there are many other people that have asked for help on the topic to no avail.
As you can see on the screenshot that Jérome sent you :
pasteboard.co/GCVxGN0.png
The Canada Post shipping platform is returning the error message "Merchant CPC ID not found on server.", which means that you either didn't properly set your Canada Post Merchant ID through the "Merchant id" option of your Canada Post shipping configuration page, or that your Merchant ID isn't compatible with their old API.
It's actually ridiculous that the only solution to this basic problem is to buy a $127 plugin
The price of the "Canada Post (Advanced)" shipping plugin is actually 45 €.
Also, we can't really help if their platform is only returning the "Merchant CPC ID not found on server." error message with your merchant ID, when we test it on our end with a valid Merchant ID, it's working fine.