mac mini late 2012 running Mojave says up to date and won't update to catalina

Check in your Applications Folder for the "Install macOS Catalina" installer.

If you want to Upgrade, open "Install macOS Catalina" and install 10.15 as per the instructions.

If not want Catalina, moving the "Install macOS Catalina" in your Applications Folder to the Trash and Empty the Trash.

If there is no "Install macOS Catalina" installer in your Applications Folder and you want to upgrade?

see > How to upgrade to macOS Catalina - Apple Support

Should you not have investigated and resolved potential upgrade issues where 32-bit

and 64-bit questions will affect a later macOS, you should do so before Catalina.

See Go 64 and read about old depreciated 32-bit drivers and their associated software/hardware:

• Go64: https://www.stclairsoft.com/Go64/

Mojave 10.14.x was the last macOS to use 32-bit drivers, and support software for older hardware.

• We Recommend You Delay Upgrading to macOS 10.15 Catalina - computer advantage

• 32-bit app compatibility with macOS High Sierra and later - Apple Support

https://support.apple.com/en-us/HT208436