If you want to share your screen in a Live stream, you'll need to give your browser permission.
If you deny permission when prompted at the start of your stream, you will not be able to stream.
In this case, please try streaming again and grant permission to your browser this time.
Additionally, Mac users may have to add permission to their browser settings and permit screen recording in their Mac's privacy settings.
Please refer to your Mac OS's user guide for information on how to do this.
■ Change Privacy preferences on Mac - Apple Support