The host must belong to the cluster and be closed before you can open it. Only administrators can open hosts.
You may need to run egosetsudoers.sh on the host running the Platform Management Console processes to be able to open and close hosts from the Platform Management Console.
You can close a host without removing it from the cluster if you need to run diagnostic tests or troubleshoot. It can then be opened again from the Platform Management Console. Closed hosts do not accept new workload. Take a host or hosts down so that you can perform maintenance on it and return it to the cluster. Closing a host means that EGO does not allocate this resource to run workload until it is opened again.