The postman-api-key is only used to check the solution. You make a call to the Postman API to check the static data in the Collection against a set of tests - it’s not used in the course scenarios.
The x-api-key is the Request Header key and the apiKey is the variable name used when setting the Collection Variable from the response of the Register a new client request.
The section in the video regarding the API Key can be found at the 20 mins point.
