Watch the active layer. Your rounded rectangle is on the layer under the one the viewport is on. You probably should have it on a layer over the viewport.
Oh. That’s different. After selecting the Rounded Rectangle tool, turn off Fill in the Shape Style window. The fill doesn’t show in your rounded rectangle because the rectangle is under the model viewport. When I opened your file the Default layer was active and since it is higher in the stack than the viewport, the rectangle got drawn over the viewport.