To allow horizontal scrolling, the width of your artboard must be equal to the width of the layer that will be scrolled. If your Studio prototype won't scroll horizontally, the trouble is very likely caused by unequal widths for the artboard and the intended scroll layer.
Establishing horizontal scrolling
To establish horizontal scrolling:
Select your artboard and, in the Preview area of the inspector panel, set Scrolling to Horizontal or Both.
Select the scroll layer and, at the top right of the inspector panel, click the Fix Position icon ().
This will fix the scroll layer to its current position.
Ensure that the width of your scroll layer is equal to the width of its artboard.
Previewing horizontal scrolling
After completing the steps above, you can preview the artboard to test out your newly established horizontal scrolling:
At the top-right of the canvas, click the Preview () icon.
You could also use the keyboard shortcut: ⌘P
At the top-right of the Preview modal, click the settings () icon and select the Override artboard settings checkbox.
If this checkbox is not selected, the Preview viewport will be the same size as the artboard, preventing the ability to scroll—since the entire artboard would be visible.