If 2020 isn't in the version dropdown, click the (bottom-right chat icon) and type "Download Revit 2020" to request a direct link.
Autodesk has a "previous version" policy that generally supports the current release and up to three versions back. Since Revit 2020 is now several years old, it may be listed as "Legacy" or "End of Support." autodesk revit 2020 trial version download
Students and educators with a verified Autodesk Education account can often access a broader range of older versions, including 2020, for educational use. System Requirements for Revit 2020 If 2020 isn't in the version dropdown, click