let's leave postman out of the equation for a while so that we don't end up troubleshooting another software.
What is the IP address or the hostname that your elasticsearch node is accessible at ?
Do a
curl -k -u username:password -X GET https://IP_ADDRESS
-k is so that curl would not try to validate your certificate
username:password use for example the elastic user and its password assuming you also went through and did this: https://www.elastic.co/guide/en/elasticsearch/reference/current/configuring-security.html too.