Is the image JPG, PNG, or WebP? Those are the only image types available for use in Generative Fill. If it is, when you go to Generative Fill, you should see this:
If you click on "upload image" it should bring up a file selection dialogue box. Are you getting that?
After selecting the image, you would then see the editing page:
Are you seeing something different?
If you can provide any screen shots of the full browser window to show what you are seeing, that would help.