Q: How can I retrieve an admin user name?

Easy. Go to the Apple menu->System Preferences->Users&Groups.

The names of the users appear on the left

Open terminal and type whoami   it will give the admin name .

From the command line, you can list all users with admin privileges, copy and paste:

dscacheutil -q group -a name admin

kaki64 wrote:

 

I don't have the admin name, so it will not show the other users.

Did you actually try?!!!

Log in as guest (or any other account); System Preferences->Users&Groups shows the names of all the accounts.