You may experience this problem, even with the firewall off, depending on the permissions settings of a target endpoint.
RPC server is unavailable. Please allow WMI through Windows Firewall.
To fix it if this occurs, open a command line window on the endpoint (as an administrator) and enter the following:
Old command (Windows 7 and below)
netsh firewall set service RemoteAdmin enable
New commands (Windows 8 and above)
netsh advfirewall firewall set rule group="remote administration" new enable=yes
netsh advfirewall firewall set rule group="Windows Remote Management" new enable=yes
netsh advfirewall firewall set rule group="windows management instrumentation (wmi)" new enable=yes
If this occurs, after making the above changes, open up the windows firewall and look for the following:
Enable all Virtual Machine Monitoring rules if available.