Without being able to look at your system it’s hard to know for sure.
But I suspect you can’t use it because haven’t put Lounge Lizard (great vsti by the way) into a collection.
The left side of the window shows every plug-in that Cubase is able to locate on your system. However this is not the list of VSTi’s you can use in a Project.
The right side of the window shows Collections which you can create and organize however makes sense to you. When you are selecting a VSTi to use on a Track the list of available choices is determined by the Collection. This means after you install new plug-ins you will want to drag them into one or more Collections.
You probably should check out the Plug-in Manager section in the OPs Manual.