Disk Utility Erase process failed: couldn't unmount disk, in use by process
Help. I am trying to erase my iMac (21.5-inch, late 2021, running OS Catalina) before I give it away or sell it, following the Apple instructions here: Use Disk Utility to erase an Intel-based Mac - Apple Support
I've encountered this problem and I can't figure out what to do next. I'll appreciate any suggestions. I'm not super tech savvy, so please keep it simple. Thank you.
iMac 21.5″, macOS 10.15