This error can occur when EasyPost's Anti-Fraud flag is triggered for your EasyPost account. For more details on how to resolve this, see this article.
This error can also occur when the shipping amount for a shipment exceeds the maximum limit on EasyPost. To resolve this, email [email protected] and ask if you're experiencing this error due to limits and request the shipping amount threshold to be raised. Make sure to include the email address you used to create the account.