my mac mini is a late 2012, it is listed as a compatible one for MacOs Catalina but when I try to update it say it is not available. what can I do?

Your last chance to change the situation:

1) plug an external drive and MANUALLY backup your data there. Without Time Machine, or it may also transfer your bug.

2) Turn comp off. Unplug power cord for a minute, so we’ll reset SMC.

3) Turn on and immediately press and hold Command+Option+R. Hold until you see Planet logo instead of Apple. It will be an recovery over Internet.

4) Launch Disk Utility and format your drive. If you have a HDD-only config, then go to next step. If you have an SSD or Fusion Drive, then format it to APFS file system. If you have SSD+HDD, then ask me how to make a Fusion Drive from them.

5) Install OS. If it is Catalina, then it will be last step. If it is an older OS, install it, then download Catalina from Appstore and write a boot USB (16 gb). Then repeat these steps but use USB boot instead of Internet recovery.

After success don’t forget to throw back your data)

click on > https://apps.apple.com/us/app/macos-catalina/id1466841314?ls=1&mt=12 and hit the "Get" button when the App Store opens.

Apple says for some versions to update catalina will find in app store.. search in the appstore

Try to download it from the Appstore. I have 2014 mini and I had the same thing 'not available' via System Preferences. I downloaded it from Appstore from the second try it started to download