Macro which handles selected/no selected token differently?

Is there a way of making a macro behave one way if a token is selected, and a different way if not? For example, an attack macro that displays the selected token's defense score for comparison, but only if there's a selected token . If not, it just skips that step.

No, unfortunately. This is because of the roll20's order of operations. It attempts to read the value of all attribute calls (including anything using the selected| construction), and when it cant find a value, it reports an error. Since you're a Pro user, You could handle this in an API script, but you'd need to write script or have one written for you. And scripts arent very flexible, usually. They serve a specific purpose.