I have recently installed Mac OS Catalina 10.15.7 with a clean installation, Later when I tried to install VM Fusion 12.1.2
However, when I double click on the .dmg file, I get the error "VM Fusion quit Expectedly.
Attached is the screenshot of the error.
Did you verify the download?If not, have you tried to download the installer again?
Your screenshot shows 19H2 for the macOS version (this was from Sep 2020). The latest update for Catalina is 19H1217 (May 2021).
Try updating the Catalina 10.15.7 from Software Update to the latest and see whether the problem gets resolved.