I have Mac OS catalina can't download latest software why?

Hi,

Exactly which version of Catalina are you running? If it's 10.15.7, that's the latest version of Catalina, plus there have been security and Safari updates. Have you run Software Update recently?

Exactly which Mac do you have: Apple Menu > About This Mac. This will tell you if you should be able to upgrade the OS any further. See: How to get old versions of macOS - Apple Support

Even if you cannot upgrade the OS beyond Catalina, Apple has still been supporting security updates for it, so you should be safe and not need a new computer for the time being. Apple usually supports the three most current OSes, and based on the past, usually brings out a new OS in the fall, so you should be good until then.

What is your current computer?

What model (e.g., iMac, MacBook Air, etc) and what model year is it? Both of those are available at Apple menu > About This Mac.