Please make sure that you have permissions for streaming at https://www.youtube.com/features?nv=1
Please note: To live stream on YouTube, you need to have no live streaming restrictions in the past 90 days and you need to verify your channel. It takes 24 hours to activate your YouTube account for live streaming.
You can find more information on how to verify your Youtube account here.
If it is so, remove your YouTube connection, and add it back - make sure you are connecting the account you want to stream to as it's common to add Gmail account instead of YouTube account.
If that won't help - go to https://studio.youtube.com/channel/id/livestreaming/manage, open your default stream settings, reveal the key, refresh it, then remove your channel from Restream and connect again.