The repository 'https://brave-browser-apt-release.s3.brave.com focal InRelease' is not signed

Using the instructions found at https://brave-browser.readthedocs.io/en/latest/installing-brave.html#linux I reinstalled Brave onto my Ubuntu machine (Ubuntu 20.04.1 LTS; GNOME 3.36.3; 64 bit system).

How do I resolve this error message?