The installer error does not tell much, just that the proxy didn’t start. Could you please upload /var/log/foreman-proxy/proxy.log? That should help. Is this deb based on rpm based system? Would it be possible to upgrade to 1.21?
Hello, that error is probably caused by missing public ssh key of smart proxy on the target machine. It could be something else, but in general, proxy can’t ssh to the target. Double check private key permission, user foreman-proxy must be able to read it. If you don’t use default path, check selinux avc denials.