click the lock to make changes doesn't work

Hiiii, I use high sierra ,I wanted to change the administrator username, so i did. i clicked the lock and then restarted my computer,when it restarts it showed my previous username so i wrote my password and it work, then i access to user & groups and when I´m trying to change the user again i clicked the on the lock to make changes button and it won't work, because it doesn't support none of the user names (i tried the old one,the new one, EVEN my apple id), i don't have another admin acount and now i dont have a admins access to my computer, because it's locked! i already search for the house icon and tried with the name that it shows and it doesn't work,

Please please help meee😕