Just updating the status .
it might be helpful for those who are facing the same issue.
Issue 1(could not start executor) is resolved.
it was due to incorrect robot password on the orchestrator.
For unattended bot, we need to provide machines login credentials in orchestrator robot.Now Changed it to robot machine’s login password.
Issue 2 (cannot create unknown type): working on it.
I think robot is not able to download packages automatically .
we might need to white list the below.
www.myget.org, *.vo.msecnd.net, www.nuget.org (port 443)
I will test and update the result once its white listed.