I am trying to change my password but there is no account in the User Account Settings list and I can not unlock the password field.
Try changing the password in the terminal.
Open a terminal window and type:
sudo passwd USERNAME
where USERNAME is the name of the user account you want to change the password for, or (even simpler) just type
to change the password of the account you're running the command from.
When prompted, then type your current password, then the new password, and finally retype the new password.