ToolBox (Panel of Elements in the Russian edition) in Visual Studio is available only if you have a form open in design mode. When switching to any other file or form code, the ToolBox does not display items.
In addition, in the latest versions of the studio, I observed a bug with the display of ToolBox elements and menu items, the studio forgets to load something there at startup or does not have time, but does not throw errors, it just does not show part of the interface. I do not have more accurate information, I did not dig, because I met only on virtualka in terminal mode. It is treated by restarting the studio, from the second launch everything is already being uploaded.