Reference: Medium Article by Ambrose Little
I recently migrated to a new MacBook Pro. The old one was on High Sierra, but the new one wasn’t. So I created a temp “admin” account to do the upgrade, then do the migration with Migration Assistant.
All seemed okay, but when I went to delete the original user account, I couldn’t. And I don’t mean “you don’t know how to delete accounts” couldn’t. I mean, I tried every possible way, and it wouldn’t couldn’t.
I found this thread on the Apple forums with people having the same problem. After trying everything else, this is the solution shared in one of the replies that worked. Warning, I have no idea of the side effects this might have, but so far so good for me. Also, do not come crying to me if it totally hoses your machine. You’ve been warned!