"Cannot create outline" error

[undresedmonster]

Hello all-

I'm trying to apply styles to outlines, but when I click the "Create" styles button in the Outline settings dialog box, it gives me an error, "Cannot create an outline."

This occurs both when I highlight rows that are currently not outlined, AND when I highlight an existing outlined group of rows that I would like to apply or create styles for.

Its the first time I've used the outline feature, so maybe I'm missing a very important step.