Hi @kevindsp, you can try downgrading to gazebo 9 to try it out, seem like this is a versioning problem and some modules may be deprecated and changed. I’m always using 9.19 and Ubuntu 18.04, ROS melodic would download a gazebo 9 for you too.
You can run the code in terminal below
sudo apt-get remove gazebo11*
# ... check whether you added the ppa/sources list if needed
sudo sh -c 'echo "deb http://packages.osrfoundation.org/gazebo/ubuntu-stable `lsb_release -cs` main" > /etc/apt/sources.list.d/gazebo-stable.list'
wget https://packages.osrfoundation.org/gazebo.key -O - | sudo apt-key add -
sudo apt-get update
sudo apt-get install -y gazebo9 libgazebo9-dev