There are two main reasons that you could be seeing this error on your shipments. The first being that the service is unable to reach the shipment destination in within the service timeframe. You can see these timeframe limitations in the images below
Correct as of January 2020
If you would like to download the original PDF document, you can do so from here
The other possibility is that the postcode itself is invalid. In this situation I recommend double checking that the postcode is valid for the country you're trying to send too.
If you have gone through both of these methods and nothing seems to be fixing the issue, please contact support and we'll be happy to help.