Sorry for the delay to reply. I get confused when I start trying to fix this after reading numerous Apple community user comments. And I don't have access to my wife's Mac mini easily when I ask her not to use it for a while.
I had gone into Recovery Mode as jchuzi explained.
Then I double clicked the Disk Utility icon at the bottom of the Finder.
When I look at the DU window side bar, I see this list and I am not sure which one I should be selecting:
This is what I see from TOP down…
1. Apple SSD… media
2. Container Disk 3
3. MacIntosh HD - APFS Volume Group
4. MacIntosh HD - APFS System Volume APFS ( this line is greyed out)
5. MacIntosh HD - APFS Start up Snap shot
6. Data - APFS Data Volume APFS
7. Extra Volume - APFS Volume APFS (this line is all greyed out)
When I chose one (3.Mac HD - is that the right one?), and go to Menu Bar/Edit - it only shows "
ADD APFS Volume". Yet Super Duper said to "
CONVERT to APFS".
Then the message is the "conversion may fail and cannot be undone."
IF I ADD, where and how large of an addition? Every step seems to have another question. I'm sure this is simple for all you TS experts, it just makes me nervous to screw up my wife's Mac during the time she is preparing our income tax. Yadda.
And how do I know when I am in Recovery Mode or not?