Football GM only works in Chrome (version 49 or higher), Firefox (version 47 or higher), and Safari (version 10 or higher). If you're already using one of those browsers and it's still not working, first make sure you're using the latest version. If you are, then continue reading...
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If you're using Chrome, you can do even better. Copy chrome://inspect/#workers into the address bar and hit enter, click "Inspect" under https://play.football-gm.com/gen/worker.js and it will pop up another window where error messages might appear.