Delete a 2nd admin account

Hello,

Today i created a 2nd administrator account to test something with a printer installation.

Now i don't need that 2nd administrator account anymore. So i logged out from that account, logged in on my own administrator account.

I went to users and groups, clicked on the padlock on the bottom of the screen and filled in my password.

When i clicked on the new admin account i was not able to delete him because the - sign was greyed out.

Even after restarting the iMac i didn't get the possibility to delete that account. What am i doing wrong?

I cannot find my answer anywhere else on the web so i hope somebody can help me.