VST Instruments not available

I’ve used Cubase for 20 years and just upgraded to Cubase LE AI Essentials 9. Windows 10

The instruments show up in the MediaBar but I can’t do anything with them. The sounds seem to be there in Documents > Steinberg > Content > VST Sound. (Also in Downloads and Documents > Cubase > Cubase win > Additional content > VST Sounds).

I also get the message: “HALion Sonic SE: Some audio files are missing. Please check installation”. I have no idea how to “check installation”. The sounds are not in the Plug-in manager. HALion Sonic SE is there but doesn’t show any sounds.

I’ve spent 3 hours a day for 3 days on this, even reinstalled Cubase. PLEASE help if you can – I don’t know what else to do (thinking of phoning Steinberg in U.K.). Such a basic thing, I can’t believe I haven’t figured it out!