Private services are only accessible by other services in the same private network. It looks like you are trying to SSH into it from the public Internet (via ssh.oregon.render.com), which will not work.
Sorry for the delayed response. I was wrong about the SSH access earlier, it is possible to SSH into private services; however, for docker services, there are some limitations that you might run into. See the docs at https://render.com/docs/ssh#limitations, you might be able to resolve the SSH issue by configuring the container differently. Let me know if this helps!