Step 1: Message the buyer. Ask them to verify the shipping account number they provided. If the number matches what you entered, ask them to verify that their account is paid up. This is usually the problem.
Step 2: If you know that you have the buyer's correct UPS/FedEx/DHL number and their account is in good standing, contact UPS/FedEx/DHL support. Be sure to give them the error code that you're getting.