The panel with the VerticalLayoutGroup has to be the parent of the objects you want to arrange within it. In your object tree it appears to be their sibling.
When you instantiate the LevelListItem buttons, make them children of the ContentContainer, not the ScrollPannel.
It also appears like you deleted the horizontal and vertical scrollbars of the standard ScrollView (or never had them in the first place because you created the ScrollView manually instead of from the "create new object" context menu. You shouldn't do that unless you know what you are doing). Without these there is no way to actually scroll it. When you want to hide them (always or only when they are unnecessary), then there is a property for that on the scrollbars.