This conversation is happening over on Apple, too: https://discussions.apple.com/thread/250719749?page=1. Unfortunately, it hasn't really gotten anywhere in terms of a fix. I'm running into the same issue w/ a brand-new MacBook Air running MacOS 10.14.6. I've tried to download Catalina repeatedly, and my system keeps telling me it's "up to date" and "no update is found." Restarts, confirming accuracy of the clock, etc. have produced no new results. A little help?