In ArcMap, I am trying to remove some statistical areas manually from a layer of statistical areas. To this end, I am using the Editor Toolbar. But when I try to choose which layer to edit, it won't let me click "ok". What may be causing this? Is there a more simple way to proceed?

The following screenshot is what I see when I try to allow editing of the statistical areas layer. The "ok" button does not turn black (i.e. is not clickable) no matter which layer I choose.