Unit firewalld.service could not be found.

This error message means that the firewalld service is not installed on your system or it is not running. Firewalld is a firewall management tool for Linux-based systems. You can check whether firewalld is installed on your system by running the following command:

sudo systemctl status firewalld

If the output shows that the service is not found, you can install it using the package manager for your Linux distribution. For example, on a Red Hat-based system, you can install firewalld using the following command:

sudo yum install firewalld

After installing firewalld, you can start and enable it using the following commands:

sudo systemctl start firewalld
sudo systemctl enable firewalld

This will ensure that firewalld is running and will start automatically on system boot.