Can't install Gazebo 11 on Ubuntu 20.04 Focal

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After installing the instructions here for step by step install, I still can't install Gazebo11 on Ubuntu Focal. I get the following error message:

sudo apt-get install gazebo11
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package gazebo11 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'gazebo11' has no installation candidate