Why can I not download macOS catalinia 10.15

If you are running a version of OS X prior to Mavericks (10.9), then you cannot download Catalina from the Mac App Store. If your Mac fails to meet the hardware requirements for Catalina, that would be another reason. If your hard drive is nearly exhausted and a full download of Catalina would fail, that is another reason. Attempting to download Catalina while signed into the Mac App Store with an Apple ID different than associated with the current operating system would be another reason.