Reason that button is locked
The Android has been equipped with the feature called "Alert Window" or "Toast". It is the ability to display the layout in front than normal app.
If this feature is exploited, a user will be allowed to unintended operation named "Tap Jacking Attack".
Android has set the security to some buttons in order to prevent this. If the layout in front of the button is being displayed, the button is locked.
Generally, the security is released by stopping the app that displays the layout in front of the button.
About Tap Jacking Attack
Illegal window is displayed in the foreground and a user will be allowed to illegal operation as follows:
・Download and install the illegal apps
・Fraudulent billing
・Grant Illegal permission, etc.
About Locked Button
Locked button will not change the color even if the tap.
In Android 6.0 or later, the following message may be displayed when you tap the locked button.
・Because an app is obscuring a permission request, Settings can’t verify your response.
・Screen overlay detected To change this permission setting, you first have to turn off the screen overlay from Settings > Apps