APFS

What year model Mac Mini?

What macOS is it currently running?

Does it have a Mechanical Hard Drive or Solid State Drive?

If you are erasing and reinstalling macOS, see > How to erase a disk for Mac - Apple Support

Mojave should have already set it to APFS if you re-installed or upgraded from an earlier macOS.

Otherwise, you will need to have a Time Machine backup, erase the drive and format it as per the instructions that I posted above, reinstall macOS Mojave and then migrate your data back from the Time Machine backup.