I would disagree about admin accounts hidden from view. I suspect the OP account used to be an admin account, but was reverted to a standard account (so there is no admin user account) I had this happen to ME on a 10.13.3 update. My account, the only account on my Mac, was changed by the update to a standard user. Only one user, as a standard account. You can't do much of anything if there is no admin account, and the root user is not yet activated - which is what happened to me.
So, the problem is - you don't have an admin user.
This occasionally happens with a system update, where an admin user is changed by a glitch to a standard account.
Now you have a login password, but you can't change or install anything that requires admin credentials (because there is no admin account.)
One way to fix this is to enable the root user, then log in to the root account, where you can change your usual user from a standard user to an admin user.
You can enable the root user while booted to single-user mode. There's other methods, but this is one that I use, and I just checked to be sure it works from High Sierra.
Restart, holding cmd-s.
When you see the text scroll down, type /sbin/mount -uw /
Press enter, then type launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist
Press enter. You may get a prompt, or maybe a couple of lines of text, but type passwd root
Press enter. It should ask for "new password", so type a nice password for the root account, and press enter.
You will NOT see your password as you type it in (it's a security feature in the terminal so no one can find out your password by watching the screen as you type it in.
)
Press enter, and enter your new password again
Finally, press enter
You may see a few lines that you can ignore.
Type exit, then press enter. You should log in to your usual account.
NOW, log out
you will see a new user, named Other. Choose Other, then enter your new password, and log in.
And, you are the (powerful) root, or superuser. You can do lots of "stuff" with your system, and no password is needed while you are logged in as root.
So, open System Preferences, then Users & Groups. Click on your usual user (whatever it might be), and click on the box "Allow user to administer this computer", and then quit System Preferences.
Log out, then choose your usual user account, enter your usual password, and log in.
And, you are back in business with a working admin account (and you should be able to install, etc, using that password now, with the password that you normally use. (The new password is ONLY to log in to your root user, which you shouldn't need normally)
The normal recommendation is to now disable the root user. You can do that easily now, through the menus in your Directory Utility. (that would not work before, because you didn't have a working admin password)
You should be good now...
