iMac can’t erase volume because kernel 0 is using it!

Let's try this (this will erase all data):

1. Shut down the Mac.
2. Turn on the Mac, and immediately press and hold Option + Command + R until you see a spinning globe.
3. Once you are in macOS Recovery, verify you are connected to WiFi in the top right corner.
4. Go to Disk Utility.
5. Select "View" in the toolbar —> "Show All Devices".
6. Select your main hard drive or SSD. This will be the top, non indented hard drive. 
7. Click Erase, and input the following exactly: (If you cannot select erase on the main hard drive, erase the "Container" or “Macintosh HD”.)
8. Name: Macintosh HD
9. Format: APFS
10. Scheme: GUID Partition Map (if shown)
11. Click Erase. If “Erase Volume Group” is shown, use that instead. 
12. Quit Disk Utility
13. Go to the reinstall macOS app.
14. Follow the prompts, and install on Macintosh HD.

Cheers,

Jack