The EW page lists 2021 as supported.
Incorrect, you get it from the EW, but you open it in an external web browser and download the RBZ manually. Then you install it via SketchUp’s Extension Manager by clicking the blue “Install Extension” button (at the bottom left of the EM,) navigate to where you saved the RBZ archive, select it and click Open. The extension should be installed. Usually, you would restart SketchUp so that the new extension loads normally with all the rest.