Make sure polkit agent is running or run the application as superuser.

This error message usually appears when trying to perform a privileged action without the necessary permissions. Polkit is a system service that manages policies for unprivileged processes, and the polkit agent is the user interface that prompts the user for authentication when necessary.

To resolve this issue, you can either start the polkit agent or run the application as superuser (with administrative privileges). To start the polkit agent, you can try running the following command in the terminal:

/usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1

If that doesn't work or you prefer to run the application as superuser, you can use the `sudo` command followed by the application's command. For example:

sudo gedit /etc/fstab

This will open the `/etc/fstab` file in the Gedit text editor with administrative privileges. Just be careful when running applications as superuser, as they can potentially cause damage to your system if used improperly.