Failed to restart ntpd.service: Unit ntpd.service not found.

This error occurs when the ntpd service is not installed on your system or it is not recognized by the init system. To fix this issue, you can try the following steps:

1. Check if the ntpd package is installed on your system by running the command:

   dpkg -l | grep ntp

If the package is not installed, you can install it using the appropriate package manager for your distribution. For example, on Debian or Ubuntu, you can use:

   sudo apt-get install ntp

2. If the package is already installed, try restarting the service using the correct service name. On some systems, it might be called `ntp` instead of `ntpd`. You can check the available services by running:

   systemctl list-unit-files | grep ntp

Once you identify the correct service name, restart it using:

   sudo systemctl restart <service-name>

If none of the above steps work and you are still experiencing issues, please provide more details about your system and the steps you have already taken to troubleshoot the problem.