USERS AND GROUPS advanced options cannot access
Changing an account name in the Users &amp; Groups preferences pane is (as you've found out the hard way) not recommended unless there already exists on the system another  account with Admin level permissions so the resulting issues can be fixed.  You might be able to resolve this by activating the root account following instructions given here:
https://support.apple.com/en-us/HT204012, then editing the sudoers file using vi in terminal to add your current username as shown below.
(from my sudoers file:)
User privilege specification
root    ALL=(ALL) ALL
%admin  ALL=(ALL) ALL
[new username][doublespace here]ALL=(ALL) ALL 

Save the edited sudoers file then restart and see if using the new username account to login &amp; make any other fixes to permissions that might be necessary.