how to configure apache server to talk to HTTPS backend server?

In my case, my server was configured to work only in https mode, and error occured when I try to access http mode. So changing http://my-service to https://my-service helped.

Your server tells you exactly what you need : [Hint: SSLProxyEngine]

You need to add that directive to your VirtualHost before the Proxy directives :

SSLProxyEngine on
ProxyPass /primary/store https://localhost:9763/store/
ProxyPassReverse /primary/store https://localhost:9763/store/

See the doc for more detail.