This happens from time to time, due to a number of factors:
(1) Canada Post may be experiencing server issues.
(2) The dimensions you entered are not accurate. Double-check that you didn't forget to change 'cm' to 'in' (same with weight).
(3) The address may need a slight adjustment. Double-check the street and postal code.
Granted, if it's happening for multiple orders, it's likely the result of a technical issue. It usually fixes itself if you check back again later, but you can also reach out to Shippo's support team in the meantime. In my experience, they're pretty good about investigating errors like this in a timely fashion.