Kong: Name resolution failed

Sometimes when calling a service on kong you might get a response like this

HTTP/1.1 503 Service Temporarily Unavailable
Connection: keep-alive
Content-Length: 36
Content-Type: application/json; charset=utf-8
Date: Thu, 02 Apr 2020 09:28:06 GMT
Server: kong/2.0.1
X-Kong-Response-Latency: 12

{
    "message": "name resolution failed"
}

What does it mean?

Kong cannot access the upstream

Make sure the corret protocol http or https is used.
Also if the upstream's SSL cannot be verified then