After using dcdiag tool (e.g. “dcdiag.exe /V /C /D /E /s:dcname”,  the following error message may appear: “Could not be queried, error 0x6ba "The RPC server is unavailable.”

Usually, this issue occurred due to Microsoft Windows Firewall forbidding unhallowed TCP/IP traffic.  To resolve this issue, use one of the following technics:

1. Disable the Microsoft Windows Firewall (Less recommended).

2. Add "Remote Administration", as a firewall exception.

3. Add the following TCP/IP Ports: Configuring System Service Firewall Rules ,  as a firewall exception.

Note: Third party applications (e.g. Antivirus etc.)  may also block the TCP/IP traffic.