I can't modify Security settings of OS
Applications which interfere with screen appearance can prevent a user from makes changes to security settings while the app is active.

The operating system has to be careful to prevent a 3rd-party application from having influence over security settings by changing the appearance of security options to the user. (best guess speculation)

A screen-color-changing app, Twilight, may have been active, and therefore, from a Security Manager's point of view, able to influence a user's input to system security settings menus.