How to login again in Users and Groups again and also how to gain sudo access which lost privilege

Question

macOS High Sierra (10.13.6)

How to login again in Users and Groups Also before with old username, I could access sudo. How to gain back sudo access. I went into Users and Groups --> Unlocked successfully --> Changed Account Name --> Restarted mac.

• tried to get sudo access to one of the hidden files. got error [myname] is not in the sudoers file. This incident will be reported.
• cant logo in my Users and Groups again

I have also tried su -, vi /etc/sudoers, sudo -v, sudo dscl .

-merge /Groups/admin GroupMembership <username>
Password:
<username> is not in the sudoers file.  This incident will be reported.

Also tired

dscl . -append /groups/admin GroupMembership USERNAME
<main> attribute status: eDSPermissionError
<dscl_cmd> DS Error: -14120 (eDSPermissionError)

ok just found out i have two users

 su OLACCOUNTNAME
    bash-3.2$ sudo dscl / -append /Groups/admin GroupMembership OLACCOUNTNAME
    OLACCOUNTNAME is not in the sudoers file.  This incident will be reported.
dscl . -read /groups/admin GroupMembership
GroupMembership: root


su NEWACCOUNTNAME
Password:
su: Sorry

*It never asked me to change password or add a new one for this user.

su OLDACCOUNTNAME
id -a
uid=501(OLDACCOUNTNAME) gid=20(staff) groups=20(staff),701(com.apple.sharepoint.group.1),12(everyone),61(localaccounts),100(_lpoperator)

dscl . list /Users | grep -v '_'
daemon
blindshot
nobody
root