I had the same with High Sierra and spend the whole day yesterday to solve the problem ..
So firstly you have to update to Mojave (there is the link where you can find it:
https://support.apple.com/en-us/HT210190).
After update (in Mojave) you can find Software update: Apple logo (left, top corner) -> System preferences -> Software update.
There you find updating to macOC Catalina, BUT... it isn't ending of problems ..
I met the next one - I had "null". If you have also so there would be the second part of solving the problem (look at the screenshot below)

Now downloading works, but afterwards it wanted to download again, again, again ...
After hours I found the solution - you have to click "Upgrade Now" in Software Update, but of course it wouldn't work.
Next go to App Store, find macOS Catalina, click "GET" and .. hmm it redirects you to Software Update, but also opens "Install macOS Catalina" ... welcome home