Bernie Szukalski writes:
When you add web maps to your story map, the scale bar is omitted. Why? The scale bar may be unneeded for your story, and the map and its context may provide all you need. Another reason is that at small scales the scale bar is ambiguous; the scale will differ within the visible extent of the map. At large scales (zoomed in) the scale is accurate across the visible map. When you want a map with a scale bar, there are a couple of options you can use.
Read the blog post...
This blog post is also available as an interactive tutorial created with Story Map Journal.