Certificate Status tell you if you are credentials are active in Sandbox environment for those APIs which require Mutual SSL Authentication. It is usually blank for x-pay-token related APIs and for Visa checkout it will show as blank. It doesnt affect the functionality of Visa Checkout.
Details on how to use the API Key and Shared Secret are available in getting started guide in the below link. Check Authentication menu in the below link for samples connecting via x-pay-token.