When I flattened the image, I was able to draw the rectangle. Maybe there are other ways of going about it, but flattening worked for me.
What was surprising was that any action, other than just drawing the rectangle, was necessary at all. Put differently, what is surprising to me, a casual user, is that if I paste an image from the clipboard and want to draw on it, I have to think about deselecting pixels or choosing layers. Paste, use the tools to draw a shape, see the shape. That was my expectation. I think it was a reasonable one.