Welcome to Cubase and Computers. Are you on PC or Mac??
Anyway for your Halion 6 issue it should be uninstalled or deleted from the VST folder.
Delete Halion 6 only.
Do not delete Halion SE…
Read this also:
For your groove agent issue if you installed it properly and it’s in Cubase it should be fine.
Right click on beat agent and load kit or kit and pattern like in the image / link:
If you don’t see all your kits reinstall GA4.
Good luck.