I'm not able to join a goal
Usually this seems to happen when you've just joined the habit, but the server response required some time to catch up and display it on your dashboard. When you go back to join it again, it doesn't display the "Join" button because in fact you've already joined the goal; it just doesn't appear on your dashboard yet.
If you give it some time and it still doesn't show up, remove the app from your device and then reinstall it. That seems to resolve most unusual problems like this. If you're using the web version, try clearing your browser cache.