missing vanessa release file

I am a novice in this area running Mint21.
On following the installation instructions after running apt-get update the following error is reported.
The repository ‘Index of /gazebo/ubuntu-stable/ vanessa Release’ does not have a Release file.
I notice a similar issue for ROS and a response implied ROS does not support current issue of UBUNTU!!
I am sure this cannot be correct.
Please advise.
P.S. keyring installation seemed to go smoothly.

Vanessa is a release of Linux Mint. There is no official support for Mint from Gazebo or ROS. However, each Mint release is based on an Ubuntu release. So if you replace vanessa with the proper Ubuntu release name, it should work.