There is a security feature that when you have a screen overlay present, android won't let you change sensitive settings. The idea is that a malevolent app could display a benign-looking screen over the top enticing you to click but it may be tricking you into clicking on something underneath that will give more permissions than you intend or increase vulnerability that you did not intend. Normally you get an error message popup if a screen is overlay detected while you are adjusting sensitive settings, but maybe the error is missing for some reason. I would double check to see if there is any screen overlay app within running processes. Two types of apps generally have screenoverlay permission
1 app to tint the screen, for instance Twilight
2 app that displays floating helper panel partly obscuring other app running in foreground
You can also check which apps have permission to display overlay in settings