If you are not seeing an update to Mojave, there is a chance your system doesn't support it.
Can you provide the 3rd line from the 'About this Mac' screen on your system?
You can reach it by clicking the Apple menu at the top left, then select About this Mac.
You should see something that says something like "Mac mini (Late 2012)" only with your system model and year code, that way you can compare it against the Mojave requirements.
You can find those at...
https://support.apple.com/kb/SP777?locale=en_US
Edit:
Mac Hardware Requirements
For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:
- MacBook (Early 2015 or newer)
- MacBook Air (Mid 2012 or newer)
- MacBook Pro (Mid 2012 or newer)
- Mac mini (Late 2012 or newer)
- iMac (Late 2012 or newer)
- iMac Pro (2017)
- Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)
Hope this helps!