NETWORK_INFRASTRUCTURES_REGION_INVALID error enabling flexiblePostEgress on AEM Cloud

I am trying to enable flexiblePostEgress using the Network infrastructure API, but getting the following error:
POST Request : https://cloudmanager.adobe.io/api/program/{{programId}}/networkInfrastructures

Request Body 

{
"kind": "flexiblePortEgress",
"region": "region-name"
}

 Response Body

{
    "type": "http://ns.adobe.com/adobecloud/network-infrastructure-generic-exception",
    "status": 400,
    "title": "Network infrastructure exception",
    "errors": [
        {
            "code": "NETWORK_INFRASTRUCTURES_REGION_INVALID",
            "message": "Region is unsupported"
        }
    ]
}

Any specific reason what might have caused this issue and how could it be resolved ? Thanks in advance !!