Hi, @StanFL, and welcome. I just fired up my Ubuntu 18.04 VM, updated it, including kernel, and tried the instructions for installing Brave. It worked, so let’s take a peek at your sources list that is flagged as having a malformed entry by doing
sudo less /etc/apt/sources.list.d/brave-browser-release.list
Make sure there’s no stray characters if you copy and paste. Less should give you a single entry like this (type q to get out of it):
deb [arch=amd64] https://brave-browser-apt-release.s3.brave.com/ stable main
Is that what you get?