First off, make sure you entered the serial number correctly. For example, you need to include dashes.
This message often comes up when you are trying to activate an older version of Beauty Box with a newer serial number. Sometimes the installer doesn’t delete older versions. Close your video editing app. Then check that the Plugins folder has only ONE version of Beauty Box. Manually delete the older one if there are two Beauty Box folders.
Restart your host app. Re-entering the serial number should then activate the plugin.
If you’re not sure where the Plugins folder is, you can search for “Beauty Box” on your system and check if multiple versions are installed.