Thanks for using the addon and for the feedback, we are actively tracking all issues at https://github.com/gurumukhi/youtube-screenshot/issues/. However, it looks like an temporary issue to me, can you please retry? If it still persists, can you please provide more details about this issue (your browser version etc) on the github issues so that we can debug it further?

Thank you once again for using the app and providing your feedback. We hope to keep improving it further.