Two kinds of root makes things confusing.
The "root" of the filesystem is the highest level on the tree, and has the nondescript name of "/". By contrast the root user has the username of "root", and has a directory mounted at the root of the filesystem--namely "/root". So you might say that root's home is at /--which is the root of the file system. Therefore, root's home is at the root.
You want to make a directory at the top of the tree, which is "/". Two choices:
cd to "/", then "mkdir <dirname>"
(much simpler if you avoid the "r word".......