This error usually comes up when there are issues with your payment method, in which case, you will need to contact your bank.
However, you should also make sure that your Express 1 account is properly set up. In order to do that, you will need to follow the below steps:
1. Go to Settings,
2. Then, click on Shipping Carriers,
3. Click Purchase Postage, next to Express 1
This should take you to your Express 1 account, where you should go to your management page to make sure everything is properly set up.