Catalina

Other postings on this topic of challenges with installing Catalina, advise to remove any 'virus protection applications' as not only possibly interfering but also unnecessary on the Mac.

Personally, others can say with more knowledge if it is true, but I have an ad block app that comes on before anything else when I log in and I quit it before doing any app or security updates and for sure before an OS upgrade.

I wonder if extensions being disabled before doing an upgrade and then enabled after the OS upgrade is completed is advisable as a step, since you are having challenges with downloading Catalina?

I suppose that you know from the specifications on this upgrade, that only 64 bit applications (and not any 32 bit applications) will work on Catalina.

https://discussions.apple.com/thread/250760281?answerId=251444398022#251444398022

Do you have the required amount of storage space to upgrade to the Catalina OS?