Please give the detailed procedure to install Avogadro 1.2 on Ubuntu 20.04 and Ubuntu 22.04
Avogadro 1.2 no longer is maintained, nor any developed further.
For the development branch leading to version Avogadro, either use apt and package manager’s front end of your preference, and more recent versions packaged for your Linux distribution (tracker).
The project is hosted on GitHub with stable releases available from the release page; for Linux, the AppImage is suitable. The daily rebuild AppImage is available from the project’s landing page. In both case, download the AppImage, provide the executable bit (chmod u+x), and run the executable.