Fill button doesn't always work with generative fill. It silently fails without an error message

I want to do somethign with generative fill, but the “Fill” button doesn’t do anything. Enter doesn’t work. It’s not intuitive how to make the feature work. It should at least provide an error message saying “your prompt is not accurate”, or “you haven’t selected a valid image”, or “this type of object doesn’t support this feature”. It’s very difficult to use it.