Taking a quick look at that path, in 3.7.2 it’s telling to create a custom layout, and the idea is that the scroll container is placed on the layout, not in a snippet. (The path tells you to add a snippet in the – to be added – top region of that scroll container.)
Snippets do not support scroll containers, only layouts and pages do.
Maybe you’re trying to do something more than the learning path tells you? If so, what are you exactly trying to accomplish?