This is due to some changes on their end. We recently released a service update that will to clarify that “this shipment can only be viewed online.” This update should install automatically soon if it hasn’t already. We’re hoping to fix this properly in the future, but we don’t have any additional information at this time.
In the meantime, their tracking page should include an alternate tracking number that usually works with one of the other services Deliveries supports. There’s a box on the side of the page (or at the bottom on a mobile device) with various numbers—DHL eCommerce, Customer Confirmation, and then the one you want. It may be labeled “Delivery by Postal provider”, “Delivery by USPS Int’l”, or something similar. Often this is a number for the U.S. Postal Service or another service we support, so you can copy that number and add it instead.