I hope to be able to solve this issue for you and it seems to stem (in my case anyway) from using migration assistant. If you install the new operating system, create a new user account and login and then migrate there seems to be some sort of conflict. Quite what that conflict is I am not sure but I think it relates to the UID/USER ID and password.
You could try this and let me know if it works for you to help anyone else should they run into this issue.
1. LOG OUT of all accounts.
2. LOG IN to the account you wish to delete.
3. CHOOSE SYSTEM PREFERENCES>USERS & GROUPS
4. Click the Padlock and Authenticate.
5. Choose the account in the list that YOU WISH TO KEEP. Remember you should be currently logged into the account you wish to delete.
6. Click CHANGE PASSWORD and change the password of the account which you wish to keep to something else other than that which you were using.
7. LOG OUT of the account.
8. LOG BACK IN to the account you wish to keep with your new password.
9. GO BACK TO SYSTEM PREFERENCES>USERS & GROUPS, click the padlock, authenticate and hopefully when you click on the account you wish to delete the minus symbol will be available and you can simply remove the account.
10. Change your password back to your usual password if thats what you want to do.
Again please do let people know if this works to help others with the same issue. Similarly if it doesn't work for you please also let post and let people know.
Good luck and hopefully this will do the trick for you and any other people running into this issue.
All the best. 😉
Stefan
