Apigee fault resolution: messaging.adaptors.http.flow.NoActiveTargets

If the proxy is using Target Servers, you may encounter the error below:

{
    "fault": {
        "faultstring": "The Service is temporarily unavailable",
        "detail": {
            "errorcode": "messaging.adaptors.http.flow.NoActiveTargets"
        }
    }
}

This issue occurs when the target server configured is either disabled or wrongly configured.

To resolve the fault, the entry should be Enabled or configured properly.

(quite a simple resolution right? but it seems easily missed out)