I could not find any errors. I was able to turn Autograde on and off with my personal account.
If you make a copy of the quiz and put it in your persona drive, I'm guessing you will be able to turn Autograde on. It this doesn't work, try from your home computer.
The incognito window is usually available as an option under the three dots in the upper right corner of the screen. If the school has blocked an incognito window, they may have blocked other stuff.
I'm happy that you are using Flubaroo but sad that you are having issues. I have to guess they are school security related.