The code is working for me. Are you sure you've used the correct server relative URL for the folder in your code?
In my test code it was formatted like this
/sites/Demo4/Lists/Folder Test/Folder 2
Also, if you want to reference the root folder of the list or library, use the RootFolder property of the List object rather than the GetFolderByServerRelativeUrl ...
There is no SharePoint out of the box way to organize lists/libraries in folders under the site contents.
However, to find the list/library easily you can try these ways:
Sort lists/libraries based on Name, Type & Number of Items like:
Similar thread: Is it possible to organize the site contents page?