According to the Amazon documentation, you simply need to accept the terms of conditions in your Shipping Services section of Seller Central.

Here is a snippet from Amazon:

Terms and conditions

Before a seller can use any application that uses the Merchant Fulfillment service to fulfill Prime orders, they need to:

Sellers can review and accept terms and conditions on the Manage Shipping Services page of Seller Central. Calling the Merchant Fulfillment service on behalf of a seller who has not accepted the terms and conditions of Amazon and the carrier returns the TermsAndConditionsNotAccepted error.