When you say you downloaded Halion VSTs, do you mean you installed the full Halion installer and it’s VSTs or just the content that is included with Cubase which is the Halion Sonic SE version?
If you installed the full version of Halion without purchasing the separate full or trial license you would get that error. So, either uninstall it (you will still have the Halion Sonic SE version that is included with Cubase Elements 10) or download the trial license for it here.