This error can be caused by exceeding the standard limit of requests per minute from Canada Post's servers. It may also occur if Canada Post is experiencing a service outage or undergoing scheduled maintenance.
In most cases, you can typically ship again without issue just waiting a few minutes.
Canada Post normally allows 20 requests per minute via API. This would come down to 5-10 shipments/minute at worst depending on what requests need to be to sent.
Pre-selecting rates instead of choosing to shop for rates before each shipment saves one request per shipment. Shipping internationally may require requesting the label and possibly a customs form if they provide one.
If you encounter this often, you can request a limit increase on your Canada Post account by contacting Canada Post: https://www.canadapost.ca/cpotools/apps/drc/increaseLimits?execution=e1s1