For users receiving the "Unable to generate key" message:
You must upload at least one logo for your progressive web app to be installable.
I just tried it at FreeFlarum and it resolved the error. It's also mentioned at the extension page, but I think that many people didn't realize that. I think that this should be put "more on eyes" (maybe in a warning box when the key fails to generate). Or there should be a condition to check whether or not at least one image is uploaded, and show a warning based on that.
Edit: It seems that this doesn't always work. Not sure what is causing this