To download (version 17.x) for Mac, you must use the official Adobe Creative Cloud Desktop app. How to Download the 2020 Version
Adobe generally supports the current major release and the one immediately prior (the "N-1" policy). Because After Effects 2020 is now considered a legacy version (older than N-1 as of April 2026), it may not appear by default. Instal older version of After Effects - Adobe Community Adobe After Effects 2020 for Mac Download