Does it happen on all websites or just one? If just one, maybe the website is broken. Try to uncheck "Use the manifest for determining app properties" when installing it. If it happens on all websites, please open an issue on GitHub.

Support for installing arbitrary websites has been added in 0.4.0. This extension now supports basically any website that Chrome and Edge do.

The address bar button still won't appear for such sites (but it also doesn't in Chrome and Edge), but you can install them from the main extension widget/button.