getting error while sending request via API_KEY

Hi Team,

I have published a API to api portal and mediator via centrasite.

Centrasite has configured the API_KEY as well. while testing in mediator passing API_KEY in http header we are getting the following error.

Mediator encountered an error:Either a required header is missing in the request or the request cannot be authenticated or the request fails validation in mediator while executing operation:getVehicleRegistration service:Vehicle_Poc_VS at time:10:43:39 AM on date:Oct 25, 2017. The client ip was: The current user:Default. The consumer application:null ----TEST MESSAGE

The same we have tested through API portal got success.

Could you please help me in this case.

question 1) How API portal will consider this request is having API_KEY?
question 2) Why mediator is not accepting when passing the API_KEY in the http header.