Trying to make changes in Users and Groups and getting Error - Help /: [Video]

[benyben123]

Hi all.
I fresh installed MacOs High Sierra. Great.
I set a Username and Password (which I remember!).
I then decided to change the Password. It worked. I can now go through the Lock Screen with my new pass (3 digits).

Ok cool.

But just now I tried to install VLC, and the Mac wants me to confirm this as safe.

So I go to Users and Groups and I am clocking the LOCK icon, I insert my Username and Pass - and ERORR! Won't recognize!
What? Why?

Thanks so much for the help!