My Mac has No Admin account
My Mac has just one Standard user account (no additional account else).
I've had only one account which was the administrator of this Mac.
Recently I changed the account's username from "user" to "A" as I went to Users&Groups.
However after I close the Advanced Option window, I didn't get the change.
My account name is still "user", but the account was changed from Administrator to Standard.
Now I lost all the Admin control so I can't unlock the Users&Groups and of course, I can't allow this standard account to be admin again or change the name.
I already tried to unlock with username "A" or "user" but I failed.
I guess it made some error while changing the username and took my admin control away...
How can I get the admin control back? Can anyone help this problem?
iMac, macOS High Sierra (10.13.1)