When I add the domain controller, I get an error "The given servers are not operational".

Error message:

When I add the domain controller, I get an error: "The given servers are not operational."

Possible cause:

1. Domain Controllers (DCs) are down.
2. The firewall has been enabled, and port 389 is closed.
3. The network is busy.

Solution:

1. Ping to check if the domain controller is operational.
2. Check the firewall for inbound and outbound rules.
3. Rectify any problems with the domain controller.

Reference: https://docs.microsoft.com/en-us/troubleshoot/windows-server/identity/fail-to-configure-server-using-server-manager