Autodesk no longer sells perpetual licenses for 2020. You cannot legally buy a new permanent license for Civil 3D 2020 today. Your only legal option to own 2020 is a monthly subscription to the current version (2024/2025) which allows you to to 2020 format.
For more in-depth guides on installation and software management, check out these resources. Installation Help System Requirements Community Forums Download & Setup Guides Autodesk Support Portal