How to use QGIS plugin "Rectangles, ovals digitizing tools"

I installed the Rectangles, ovals digitizing tools-plugin, found it on the toolbar, but it's inactive. I made a layer to which I want to add rectangles, it's visible and editable... The plugin bar is grey and inactive.

What to do?

Answer

The Rectangles Ovals Digitizing plugin only works for polygon layers, not for point or line layers.