What can cause "Unauthorized" error message within API Explorer?

In API Explorer within vCenter running 6.5 and 7.0 VMWare hypervisor regardless of which REST API function executed I receive an "Unauthorized" error (see details below).  I have administrative access to the environment.  

What level of administrative privileges are required to execute the RISK API functions within API Explorer?

Is there a setting within the environment that must be configured to authorize CURL statements?

This is my first attempt at using REST API and API Explorer.

Please advise.

Lipotech

{
    "error_type": "UNAUTHORIZED",
    "messages": [
        {
            "args": [],
            "default_message": "Unable to authorize user",
            "id": "vapi.security.authorization.invalid"
        }
    ]
}