Remove your YouTube from the Restream dashboard.
Press the Edit destinations button.
Press Edit beside the YouTube channel.
Press remove destination.
Then, generate a new stream key at https://www.youtube.com/live_dashboard.
Make sure to select the "Stream" option on the left-hand side of the YouTube dashboard.
The following page will provide you with the option to reset your stream key.
Once you've reset the key, go back to the Restream dashboard and try adding the YouTube channel again.