This problem has persisted for some users since the release in October. I'm posting it here because Google is referencing this thread.
So, for users in general that get the message "The requested version of macOS is not available"
First, make sure you are not in the beta software program. You will need to log out of that program. For me, that wasn't the issue.
There may be a problem with the sortware update preferences file that keeps track of ignored updates. Go into the terminal and run
sudo softwareupdate --reset-ignored
Then restart your mac, go to software update in system preferences and see if you have the option to install Cataline.
Disclaimer: I'm not smart enough to have figured this out. Thanks to Apple support. I had the same problem as everyone here. So on a lark I went to the Apple website, clicked on downloaed problems. Within seconds I get a call, then go on hold for 30 seconds or so, and this really nice and really patient Apple support staff is on the line. On a Sunday!!! We walked through the problem, repeating steps. Once the softwareupdate utility was run it was fixed. Yippee for Apple support!! If this had not worked he said he would have done the network install others have described. ( Command-Option-R )