The merchant account you are using is not setup with a gateway account selected for your Secure Checkout transactions.

To setup your merchant account to accept Secure Checkout transactions follow these steps:
  1. Select Setup > Payment Capabilities
  2. Click Edit next to the merchant account you are configuring for Secure Checkout Transactions
  3. On Step 1. Account Details, scroll down to item 7. Secure Checkout Account and select a gateway account to use for your Secure Checkout tranasctions
  4. Click Next
  5. Click Finish