When running MMC with any AD management component inside it, and try to switch to another server you may notice sometimes the status shows “Unavailable”.

Most likely this is due to IPv6 not fully disabled, or not fully enabled (if you decide to run it), and you will need to update the registry value as per Microsoft article:

https://support.microsoft.com/en-us/help/929852/how-to-disable-ipv6-or-its-components-in-windows

HTH