no plug-ins in Garage Band

Using Garage Band 10.4.12 on a brand new mac book air.

According to Help, to install a plug-in, in the Plug-Ins area I need to select an empty plug-in slot. This is not possible. See screenshot.

I have uninstalled then reinstalled GB. Still I have the same problem.

Moreover, I can't find any plug-ins in the Finder so I wonder if they have even been installed.