Remove your YouTube channel from Restream dashboard.
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 they key, go back to the Restream dashboard and try adding the YouTube channel again.
Make sure you select the YouTube Events channel.
Then choose the option to "Connect YouTube Events" and link your YouTube account.