This error message is generated when your Anti-Virus blocks the installation of Blade and Soul. If you receive this message, you will need to fully uninstall your AV then attempt to install the game again. Please note that disabling your AV when receiving this error will not work, you will need to perform a full uninstall. Once you have the game fully installed on your computer you can then reinstall your AV.
If you continue to experience an issue installing the client even after fully uninstalling your AV please contact support.