Turning off “Auto Blocker” did it for me
Thank you Stuart, I had the same issue
This solved the problem for me, thanks for posting this
I’m glad it helped and worked for you!
Bless your soul! My app is due tomorrow for class tomorrow and I was freaking out that I suddenly couldn’t download it 
Awesome - glad to feel the sense of community - thanks
Try doing below
Just go to Security and Privacy in your settings, scroll down to additional security settings and select Auto Blocker. I turned the whole thing off, but there’s an option for “Block software updates by USB cable” which is probably the actual setting that prevents you from turning on USB debugging. Just a heads up for anyone Googling this in the future or searching this subreddit.